On the Body-wall in Bryozoa.

By

Folke Borg,Vexio, Sweden.

With 6 Text-figures.

IN all cyclostomatous Bryozoa the body-wall of an autozoidconsists of the following parts : (1) the cystid, (2) the terminalmembrane (Harmer, 1898, p. 112), (3) the vestibule (Harmer,1898, p. I l l ) , (4) the tentacle-sheath. Proximally the tentacle-sheath passes over into the tentacles. In a former paper (1923,p. 2) I have briefly indicated the structure of the terminalmembrane and the vestibule in the Cyclostomata. It shouldbe specially noticed here that, owing to the insertion of themembranous sac described in the same paper (1923, pp. 2 ff.),the vestibule in the Cyclostomata is permanently invaginated,thus reminding one of the ' duplieature ' in the Phylacto-laemata.

In the following pages a description of the structure of thewall of the cystid in the Cyclostomata will be given.

In most Gymnolaemata the wall of the cystid is simple, con-sisting of four different layers : cuticle, calcareous layer, ecto-derm, and mesoderm ; in the Ctenostomata, however, the cal-careous layer is absent, and in many Cheilostomata the frontal,sometimes also the basal, wall of the cystid is constructed ina more complicated manner, showing a cryptocyst (Jullien,1881, p. 274) and a gymnocyst (Levinsen, 1909, p. v). I seeno reason for designating as a gymnocyst merely a c a l c i f i e dlamella, brought about ' by the transformation of a coveringmembrane ', as did Levinsen ; such a membrane I call gymno-cyst whether it is calcified or not.

584 FOLKB BOKG

As regards Cyclostomata the wall of the cystid has beeninvestigated by Vigelius (1887, pp. 237 ff.), who finds the endo-cyst to be composed of ectoderm and ' parenchymatous tissue ' ;further, by Pergens (1889, pp. 505 ff.), who dwells especiallyupon the calcareous layer ; and by Calvet (1900, pp. 163 ff.),who found all four layers mentioned.

My investigations show that in the majority of the Cyclo-stomata, viz. in the families Crisiidae, Tubuliporidae, Diasto-poridae, Entalophoridae, Corymboporidae, Prondiporidae, andFascigeridae, the wall of the cystid is simple and composed ofcuticle, calcareous layer, ectoderm, and mesoderm (Text-figs. 1,2). The first two layers are both secreted from the ectodermalepithelium. In the Crisiidae the cuticle is especially distinct(Text-fig. 2) ; here it reaches a thickness that is one-fourth orone-fifth that of the calcareous layer. In the other families, inwhich the calcareous layer is, as a rule, considerably thickerthan in the Crisiidae, the cuticle is, on the other hand, ratherthin (Text-fig. 1) ; its thickness is at the very most one-tenththat of the calcareous layer. The boundary between the twolayers is very marked. Owing to the continued secretion ofcalcareous matter from the epithelium, the calcareous layer is,both in the Crisiidae and the other families, considerably thickerin old than in young zoids. It consists of carbonate of lime,stratified into a dense network of very fine chitinous threads(cf. Text-figs. 1, 2). Ostroumoff (1886, p. 12), Vigelius (1887,p. 237), and Pergens (1889, p. 506) believe that the calcareousmatter is secreted inside the cells of the epithelium ; Levinsen(1909, p. 3) is inclined to agree with this opinion, while Calvet(1900, pp. 29, 165), on the other hand, considers that thecalcareous matter is deposited in the cuticle. The opinions ofthe authors mentioned are mainly based on investigations ofcheilostomatous forms. As regards Cyclostomata there isnothing to indicate that any deposition of calcareous matteroccurs within the epithelial cells. If one studies the secretionof calcareous matter at the distal end of a cystid, one findsthat the cells of the underlying epithelium remain quitedistinct and well separated from the calcareous layer ; through

BODY-WALL IN BRYOZOA 585

shrinkage, due to fixing, a small space is often producedbetween the two layers.

Calvet (1900, p. 166) has found that in the Cyclostomata' l'ectocyste... se montre constitue par deux feuillets distincts ',separated by a deposit of calcareous matter ' dans la regionmoyenne de l'epaisseur de la cuticule ; celle-ci disparaitrait

TEXT-FIG. 1.

S t o m a t o p o r a g r a n u l a t a (M. Edw.). Section through partof wall of cystid of autozoid. X78O. cut., cuticle; cal., cal-careous layer ; ec, ectoderm ; me. mesoderm.

completement a, ce niveau, faisant place au calcaire, et se sub-diviserait ainsi en deux feuillets '. Contrary to this statement,I have found only one cuticle-layer (Text-figs. 1,2); the cal-careous layer is always just inside this layer, between it andthe epithelium. Calvet's opinion may be explained by thefact that he has fixed his material in corrosive sublimate with20 per cent, glacial acetic acid ; when I have examined suchmaterial, I have often found most of the fine threads, penetrat-

586 FOLKE BOEG

ing the former calcareous layer, compressed into a thin filmjust outside the epithelium ; material, however, which hasbeen carefully decalcified in alcohol of a high degree madeslightly acid, does not show any such layer.

The epithelial layer of the wall of the cystid is, in a completezoid, an extremely thin, endothelium-like film (Text-figs. 1, 2),with here and there intercellular spaces. The cells have a veryirregular shape ; they often look as though forming part of themeshes of a net. Their nuclei form small protuberances project-ing into the body-cavity ; they are oval, with one of the longsides turned towards the calcareous layer (Text-figs. 1,2).

The mesodermic layer of the wall of the cystid is extremelyreduced ; it is made up of individual cells or groups of such,situated here and there close to the epithelium (Text-figs. 1,2).The shape of the cells is very irregular. The cytoplasm sendsout processes which, as a rule, connect with some of the out-growths from other mesodermic cells (cf. Text-fig. 2). Thenuclei are rounded, and, as a rule, somewhat larger than thenuclei of the ectoderm (Text-fig. 1).

At the edge of the aperture the calcareous layer ceasessuddenly, whereas the cuticle, ectoderm, and mesoderm passinto the corresponding layers of the terminal membrane.

The joints in the Crisiidae constitute a noticeable modificationof the wall of the cystid just described (Text-fig. 2). They areannular formations by means of which the distal and theproximal parts of certain zoids are articulated ; the mobilityis, however, very limited. The cuticle of the body-wall con-tinues over the joint, but, as Text-fig. 2 shows, it is torn offall round it, thus forming two irregular edges encircling it.The layer just inside this cuticle does not consist here of cal-careous matter, but of a very firm chitinous substance, whichshows more or less distinct stratification into layers parallelwith the surface (Text-fig. 2). Proximally, as well as distally,this layer adjoins the calcareous layer, the boundary linebetween the two layers being very sharply marked (Text-fig. 2).In zoaria that have been carefully decalcified, stained, andmounted whole, one can distinctly observe that the network

BODY-WALL IN BRYOZOA 587

of fine threads in the calcareous layer is continuous with thechitinous layer of the joint (cf. Text-fig. 2). The older ajoint, the thicker the chitinous layer. In old joints one caneasily see that the chitinous layer is wider on the inward side

TEXT-PIG. 2.

Cr i s ie l l a p r o d u c t a (Smitt). Longitudinal section throughjoint. x 630. cut., cuticle; cat, calcareous layer; ch.j.,chitinous joint; ec, ectoderm ; me, mesoderm.

than on the outside, the boundary between this layer and thecalcareous one thus running obliquely (Text-fig. 2). Theepithelial layer of the joint differs decidedly from that in therest of the wall of the cystid. Seen from the surface, or inlongitudinal section, the cells are elongated, practically spindle-shaped ; they are placed close together (Text-fig. 2). On cross-sections they are about cylindrical, with the nuclei in their

588 FOLKS BOEG

proximal parts.' The "mesodermic cells in the joint, like theectodermal ones, lie much closer together than in the rest of thebody-wall; they form a very distinct and tolerably dense net-work.

It has been pointed out by various authors that in the Bryozoathe so-called pores in the outer walls of the cystid are not realpores, but only places where secretion of calcareous matter hasnot occurred and where, consequently, the soft tissues areseparated from the outer medium by a cuticle only. This isquite true as regards Cyclostomata also. Many different nameshave been used in order to designate these formations ; I thinkthe term pseudopore proposed byLevinsen (1909, p. v) can beconveniently used.

Miss Eobertson (1903, p. 129) believes that in the Crisiidaethe epithelial cells of the body-wall pass through the pseudo-pores and ' spread out over the surface, forming a very thinlayer upon it '. This is by no means the case. I wish toemphasize that, as in all Cyclostomata, there is in the Crisiidaeno trace of any cellular layer outside the cuticle ; it is impossibleto detect even the slightest sign of such a layer either on livingcolonies or on fixed ones treated in different ways, stained orunstained ; nor does treatment with nitrate of silver bringany result.

The interzoidal pores in all Cyclostomata are real pores,constituting small round holes, here and there, through thecalcareous layer, between two neighbouring zoids, and thusconnecting the body-cavities of the separate zoids with eachother. Levinsen (1891, p. 253) thinks that the interzoidalpores ' correspond to the ordinary pores on the surface of thezooecia and in such a case are not pierced ' ; this conceptionis, however, erroneous. On the contrary, owing to these pores,the zoids in Cyclostomata have much more open communica-tion with each other than is the case in Cheilostomata andCtenostomata. It is quite obvious, I think, that the ' com-munication-plates ' or ' rosette-plates ' in these two groupscannot be regarded as homologous with the interzoidal poresin Cyclostomata, which, on the other hand, are formed prin-

BODY-WALL IN BRYOZOA 589

cipally in the same manner as the openings in the interzoidalwalls in Phylactolaemata. In this connexion another note-worthy fact should be observed also, viz. that, while in Cheilo-stomata and Ctenostomata the interzoidal (lateral) walls are,as a rule, double, in Oyclostomata and Phylactolaemata theyare, on the contrary, shared in common by two zoids.

In the two families Horneridae and Lichenoporidae the wallof the cystid differs very considerably from that of the Cyclo-stomata treated above (Text-figs. 3-6).

Harmer (1896, p. 91) mentions that in L i c h e n o p o r av e r r u c a r i a (Fabr.) he has observed ' in vertical sections ofstained colonies which have not been decalcified . . . a filmof stained nucleated tissue . . . passing over the outer side of thecalcareous parts of the upper surface of the colony . . . andextending up the outside of the zooecial tubes to the orificesof the latter '. Harmer's discovery has, however, remainedunheeded. Calvet (1900, p. 166), who has investigated thebody-wall of L i c h e n o p o r a h i s p i d a (Flem.), describes itas being constructed in quite the same way as in the otherCyclostomata.

As regards the Horneridae it has not up to this time beenknown that the wall of the cystid in this family differs fromthat of most other Cyclostomata. Canu and Bassler (1920,p. 796) say that in the Horneridae ' the . . . division into twoof the ectocyst has not yet been observed, although it may bequite probable ' ; the authors presume that the calcareouslayer of the body-wall might be divided in this manner, which,however, is not the case.

My investigations show that, in both families mentioned, thewall of the cystid—with the exception of the interzoidal walls,and, in the Lichenoporidae, those parts of the walls of thecystids that constitute the basal wall (the basal lamina) of thezoarium—is constructed in the following manner (Text-figs. 3-6) : the outermost layer is a cuticle, which in theLichenoporidae is almost immeasurably thin (Text-fig. 5) ;in the Horneridae, too, it is frequently difficult of observation(Text-figs. 3, 4), although in this family it is, as a rule, some-

590 FOLKB BORG

what thicker than in the former one. Immediately underneaththe cuticle there is an ectoderrnal epithelium (Text-fig. 6, ec.g.)of the same appearance and structure as the correspondinglayer in the Cyclostomata treated above. Underneath theepithelium there is a mesodermic layer (Text-fig. 6, meg.)which, here also, is extremely reduced ; in many cases it isdifficult to trace this layer with certainty, but at times itappears quite distinctly, so that there is not the slightest doubtas to its existence. Then there comes a narrow, slit-like cavity(Text-figs. 4, 6, hy.c). Next to this there is another mesodermiclayer (Text-fig. 6, mc.cr.), and then an ectodermal epithelium(Text-fig. 6S ec.cr.), both presenting the same appearance as thelayers just described. A calcareous layer (Text-fig. 6, cal.cr.)comes next ; on sections which have been carefully decalcifiedthis layer is represented by a dense network of fine threads(Text-figs. 3, 4, cr.) of the same appearance as in the otherCyclostomata. Underneath the calcareous layer there is yetanother ectodermal epithelium (Text-fig. 6), and then—nearestthe body-cavity of the zoid—a mesodermic layer (Text-fig. 6),both these layers having the same appearance as the corre-sponding layers just underneath the cuticle.

Of course it is by no means possible always to observe allthe layers just mentioned ; in the Horneridae in particular theouter layers, and especially the cuticle, are often damaged andpartly torn away, which perhaps is mainly due to the treat-ment the zoaria meet with, when the material is collected ;the bush-like and richly branched colonies of the speciesbelonging to this family are, also, more exposed to injuries thanthe wart-like zoaria in the Lichenoporidae. The slit-like cavityis frequently invisible because the two mesodermic layers havebeen pressed together (Text-figs. 3, 5) ; in some places, forinstance at the pores in the Horneridae (Text-fig. 4), it is, onthe other hand, as a rule very distinct. This cavity, which iscovered by mesoderm on the inner as well as on the outer side,and which is connected with the body-cavity of the zoids, mustbe regarded consequently as a coelomic cavity.

In both families the calcareous layer is very well developed

BODY-WALL IN BRYOZOA 591

and, as a rule, somewhat thicker than in other Cyclostornata.This is particularly true in the case of the Horneridae, where, inold zoaria, this layer may reach an astonishing thickness (cf.

TEXT-FIGS. 3, 4.

H o r n c r a l ic l ienoides (L.). Two longitudinal sections throughparts of body-wall. Photomicrographs. Both Text-figs, x 150.gy., gymnocyst; cut.g., cuticle of gymnocyst; hy.c, hypostcgalcoelomic cavity ; <•»•., cryptocj'st; JJ., pore.

Text-fig. 4). On sections through such thick walls that havebeen carefully decalcified, one often finds that the network offine threads forms several rather distinctly separate layers

NO. 280 R r

592 FOLKB BORG

(Text-fig. 4), probably indicating that the deposition of cal-careous matter has taken place during different periods. Inall probability the secretion arises both from the inner and theouter ectodermal layer surrounding the calcareous layer ; theformer process is, however, completely overshadowed by thelatter one. Thus, for instance, in a well-developed zoarium ofH o r n e r a l i c h e n o i d e s (L.) examined by me, a branchmeasures in width 0-7 mm. at a distance of 2 mm. from the top,while one of the main stems, in the proximal part of thezoarium, measures 3-2 mm. in width. Since the number ofzoids alongside each other is the same at both places measured,this considerable increase in thickness is solely due to secretionfrom the outer epithelial layer. The fact that a ' secondarythickening ' of the calcareous layer takes place in the Hor-neridae is mentioned by several authors, as for instance byWaters (1889, p. 278), although no explanation of this fact hasbeen given. It is well known, too, that the body-wall in theHorneridae and the Lichenoporidae presents, when seen fromabove, an uneven surface with ridges and furrows, quite con-trary to the condition in other Cyclostomata ; in numerousspecies of L i c h e n o p o r a bristles or spines of calcareousmatter are found also on the outside of the zoarium; and at therim of the aperture in most if not all species of L i c h e n o p o r a ,and in some of H o r n e r a as well, there are pointed protuber-ances. The origin of all these formations is of course easilyunderstood, -when one realizes that the calcareous layer iscovered with an ectodermal epithelium capable of secretingcalcareous matter.

Another consequence of this fact is that, in the two familiesmentioned, there are no pseudopores. The pores piercing thecalcareous layer in the Horneridae are real pores (Text-fig. 4, p.),by means of which the body-cavities of the zoids, on one hand,and the coelomic cavity underneath the cuticle, on the other,communicate with one another. They are thus homologousto the interzoidal pores. In the Lichenoporidae the noteworthycondition seems to exist that pores are entirely lacking in theouter walls of the zoids ; in the roof of the so-called alveoli

BODY-WALL IN BRYOZOA 593

or cancelli (cf. Harmer, 1896), on the other hand, pores arefrequently found.

In the Horneridae and the Lichenoporidae the terminalmembrane of a zoid is composed of the same layers as in otherCyclostomata (cf. my paper, 1923, p. 2). The transition fromthe wall of the cystid to the terminal membrane occurs, how-ever, in a different manner here (Text-fig. 6). It is only the

TEXT-FIG. 5.

L i c h e n o p o r a h i s p i d a (Flem.). Sagittal section through partof zoarium. Photomicrograph, x 145. gy., gymnocyst; cr.,cryptocyst; vest., vestibule; m.s., membranous sac; pip.,polypide.

cuticle with underlying ectoderm and mesoderm that passesinto the terminal membrane, but not the other layers. Onemight say that in these two families the terminal membrane ofeach zoid constitutes only a part of that portion of the body-wall that is situated outside the slit-like coelomic cavity (cf.Text-fig. 6). This cavity thus, at the edge of the aperture ofeach zoid, communicates with the body-cavity of the zoid(Text-fig. 6), although the communication is oertainly, as a rule,very narrow. The ectoderm and mesoderm covering the outerside of the calcareous layer bend inwards at the rim of the aper-

594 FOLKE BORG

ture, passing here into the corresponding layers on the innerside of the calcareous wall (Text-fig. 6). According to thedescription given, it might seem difficult to understand howthe terminal membrane, in the Horneridae and the Licheno-poridae, can have a constant location with respect to theaperture of the zoid, as in fact it has. The explanation is tobe sought in the fact that the terminal membrane and its con-tinuation, the vestibule, are anchored by means of muscles andligaments in the interior of the zoid (cf. Text-fig. 6).

Finally, it should be noted particularly that the interzoidalwalls, in the two families mentioned, have quite the samestructure as in the Cyclostomata treated above. The structureof the outer walls of the cystids, in the Horneridae and theLichenoporidae, is explained by the development of the zoariain these families. To this I will shortly return in another paper.

It is of course easy to compare the Avail of the cystid in theHorneridae and the Lichenoporidae, just described, with thatin the Cheilostomata provided with a gymnocyst and a crypto-cyst. The formations in question obviously show many similari-ties ; the question then arises if they are homologous to eachother and if perhaps one has developed from the other. In myopinion the latter possibility is absolutely out of the question ;I believe that the ' double ' wall of the cystid in each group hasoriginated entirely independently ; reasons for this will begiven in my forthcoming paper. Regarding the former ques-tion, it should first of all be pointed out that it doe3 not seemcertain that the cryptocyst has originated in the same wayeven in all the Cheilostomata where it is found (cf. Harmer,1902; Levinsen, 1909 ; and Canu and Bassler, 1920) ; theformation of the cryptocyst in this group seems, however, toneed further elucidation. The outer membrane (the cuticlewith underlying ectoderm and mesoderm) in the Horneridaeand the Lichenoporidae is, in reality, common to the wholezoarium ; in the Cheilostomata, on the other hand, it formsoriginally part of each individual zoid. The calcareous layerwith surrounding ectoderm and mesoderm in the two cyclo-stomatous families mentioned takes its origin in a common bud

BODY-WALL IN BRYOZOA 595

(' samknopp ', Smitt, 1865), the cryptocyst in the Cheilostomata,on the other hand, is formed in the various zoids. In otherrespects also, the mode of development of the zoaria makes it

TEXT-FIG. 6.

text

Diagram showing longitudinal section tlirough distal half of auto-zoid in Hornoridae and Lichenoporidae. cut.g., cuticle, ec.g.,ectoderm, meg., mesoderm of gymnocyst; nicer., mesoderm,ec.cr., ectoderm, cal.cr., calcareous layer of cryptocyst; liy.c,hypostegal coelomic cavity; vest., vestibule; at.sph., atrialsphincter; m.s., membranous sac ; t.sh., tentacle-sheath ; tent.,tentacle.

seem extremely improbable that the formations in question,in the Cyclostomata and the Cheilostomata, are homologous.In spite of this, however, I see no reason—especially consideringthe great likeness in structure and, I suppose, in function also—for not using the same terms in both cases. Hence I designate

596 FOLKE BORG

the cuticle with underlying ectoderm and mesoderm in theHorneridae and the Lichenoporidae as the g y m n o c y s t ,whereas I call the calcareous layer with surrounding ectodermand mesoderm the c r y p t o c y s t ; the cavity separating thetwo parts, gymnocyst and cryptocyst, of the body-wall, maybe termed the h y p o s t e g a l c o e l o m i c c a v i t y (cf. Jullien,1881, p. 276).

We can now discuss the question, are not other cyclostoma-tous families to be found where the structure of the body-wallis the same as in the Horneridae and the Lichenoporidae ?In the first place, then, it can be pointed out that this is notthe case in the majority of the families belonging to the Cyclo-stomata, viz. in those treated of above (Crisiidae, Tubuli-poridae, &c). On the other hand, it appears to me extremelylikely that the family Heteroporidae, which must be con-sidered as being rather nearly related to the Lichenoporidae,also has a double body-wall, consisting of gymnocyst andcryptocyst. If this is correct, it can presumably throw somelight on the real structure of the extinct sub-order Trepo-stomata, to which sub-order the family Heteroporidae has some-times been referred (cf. Gregory, 1909), and to which, in anycase, it should be rather closely akin. I am much inclined tobelieve that the body-wall in the Trepostomata had the samestructure as in the Lichenoporidae (and probably in the Hetero-poridae), but, as it is a case of fossil forms, it may be impossibleto demonstrate this positively. I hope, however, soon to havean opportunity of returning to this and allied questions.

SUMMARY.

The results obtained in this paper may be summarized asfollows :

1. The body-wall in the majority of the Cyclostomata con-sists of cuticle, calcareous layer, ectoderm, and mesoderm. Thetwo former layers are secreted from the ectoderm. The cuticleis found only outside the calcareous layer. The ectoderm andmesoderm are both strongly reduced. The so-called pores that

BODY-WALL IN BRYOZOA 597

pierce the calcareous layer are not real pores but pseudopores ;the interzoidal pores are, on the contrary, always real pores.There is never any cellular layer outside the cuticle of the body-wall.

2. In the joints that occur in the Crisiidae there is, instead ofthe calcareous layer, a firm chitinous substance forming anannular zone that is wider on the inward side than on the out-side of the joint. The cuticle is torn off all round the joint. Theectoderm and mesoderm are much better developed in the jointthan in the rest of the body-wall.

3. In the Horneridae and the Lichenoporidae the body-wall isdouble, consisting of a gymnocyst and a cryptocyst, separatedby a slit-like hypostegal coelomic cavity. The gymnocyst iscomposed of cuticle, ectoderm, and mesoderm, whereas thecryptocyst consists of a calcareous layer on both sides sur-rounded by ectoderm and mesoderm. The interzoidal walls,in the two families mentioned, are of the same structure as inall other Cyclostomes.

4. The cryptocyst in the Horneridae and the Lichenoporidaeis not, in all probability, homologous with the formationdesignated by the same name and occurring in many Cheilo-stomes.

5. On account of the structure of the body-wall there areno pseudopores in the Horneridae and the Lichenoporidae, butonly real pores. In the Lichenoporidae there are no pores atall in the walls of the autozoids.

6. It seems very probable that the structure of the body-wallin the family Heteroporidae and in the extinct sub-orderTrepostomata is the same as in the Horneridae and the Licheno-poridae.

E B F E E E N C E S .

Borg, F. (1923).—"On the Structure of Cyclostomatous Bryozoa",' Arkiv f. Zoologi', Bd. 15. Stockholm, 1923.

Calvet, L. (1900).-—"Contribution a l'histoire naturelle des Bryozoairesectoproctes marins." These. Montpellier, 1900.

Canu, F., and Bassler, B. S. (1920).—"North American Early TertiaryBryozoa ", ' United States Nat. Mus. Bull.', 106. Washington, 1920.

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Gregory, J. W. (1909).—" Catalogue of the fossil Bryozoa in the Depart-ment of Geology ", British Museum, ' The Cretaceous Bryozoa ', vol. ii.London, 1909.

Harmer, S. F. (1896).—" On the Development of Lichenopora verrucaria ",' Quart. Journ. Micr. Sci.', New Series, vol. 39. London, 1897.

(1898).—" On the Development of Tubulipora, &c", ibid., vol. 41.London, 1899.

(1902).—" On the Morphology of the Cheilostoinata ", ibid., vol. 40.London, 1903.

Jullien, J. (1881).—"Note sur une nouvelle division des Bryozoairescheilostomiens ", ' Bull. Soc. Zoologique de France ', vol. 6. Paris, 1881.

Levinsen, G. M. R. (1891).—"Polyzoa", ' Det videnskablige Udbytte afKanonbaaden "Hauchs" Togter.' IV. Copenhagen, 1891.

• (1909).—"Morphological and Systematic Studies on the Cheilo-stomatous Bryozoa." Copenhagen, 1909.

Ostroumoff, A. A. (1886).—" Contribution a. 1'etude zoologique et morpho-logique des Bryozoairea du Golfe de Sebastopol", 'Arch. Slaves deBiologie', torn. 2. Paris, 1886.

Pergens, E. (1889).—" Untersuchungen an Seebryozoen ", ' Zool. Anz.',Jahrg. 12.

Robertson, Miss A. (1903).—" Embryology and Embryonic Fission in theGenus Crisia ", ' Univ. of California Publ. Zoology ', vol. i. Berkeley,1902-5.

Smitt, F. A. (1865).—" Om Hafs-Bryozoernas utveckling och fettkrop-par", ' Ofvers. af Kgl. Vetenskapsakad:s Forhandl. Arg. 22'. Stock-hohn, 1866.

Vigelius, W. J. (1887).—" Zur Morphologie der maiinen Bryozoen",' Zool. Anz.', Jahrg. 10.

Waters, A. W. (1889).—" On some Ovicells of Cyclostomatous Bryozoa",' Journ. Linn. Soe. Zoology ', vol. 20. London, 1889.