VISUAL V

OCAB UNIT

6

AN

DR

EW

SH

AW

5T

H

Lacking a fluid filled body cavity

Organisms showing acoelomate formation include the platyhelminthes (flatworms, tapeworms etc.), the cnidarians (jellyfish and allies), and the ctenophores (comb jellies)

ACOELOMATE

n bilateral symmetry (also called plane symmetry), only one plane, called the sagittal plane, will divide an organism into roughly mirror image halves

BILATERAL SYMMETRY

an opening into the archenteron during the embryonic stages of an organism.

BLASTOPORE

hollow sphere of cells formed during an early stage of embryonic development in animals. The blastula is created when the zygote undergoes the cell division process known as cleavage.

BLASTULA

an evolutionary trend, whereby nervous tissue, over many generations, becomes concentrated toward one end of an organism. This process eventually produces a head region with sensory organs

CEPHALIZATION

fluid-filled cavity formed within the mesoderm of some animals.

coelom of an annelid

COELOM

A coelomate animal is basically a set of concentric tubes, with a gap between the gut and the outer tubes.

An earthworm is a coelomate

COELOMATES

the form of cleavage in most protostomes. It results in the developmental fate of the cells being set early in the embryo development. Each cell produced by early embryonic cleavage does not have the capacity to develop into a complete embryo.

A Caribbean Reef Squid, an example of a protostome.

DETERMINATE CLEAVAGE

Deuterostomes are distinguished by their embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus,

Sea cucumbers and other echinoderms are deuterostomes.

DEUTEROSTOMES

Pertaining to a condition in which there are two primary germ layers, such as ectoderm and endoderm.

Diploblastic animals have two germ layers: an outer ectoderm and an inner endoderm.

DIPLOBLASTIC

The "ectoderm" is one of the three primary germ cell layers in the very early embryo

ECTODERM

Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer), with the endoderm as the intermost layer

ENDODERM

a clade comprising all major animal groups except sponges

Anything but sponges are eumetazoan

EUMETAZOAN

a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar ("three-layered") structure known as the gastrula - it is, in other words, the formation of the gut.

Stages of gastrulation

GASTRULATION

a group of cells, formed during animal embryogenesis

Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges (eumetazoans and agnotozoans) produce two or three primary tissue layers

GERM LAYERS

a group of species united by morphological or physiological traits, that has given rise to another group that differs markedly from the ancestral condition, and is thus not considered part of the ancestral group.

Fish represent a grade, inasmuch as they have given rise to the land vertebrates. In fact, the three traditional classes of fish (Agnatha, Chondrichthyes and Osteichthyes) all represent evolutionary grades.

GRADE (PHYLOGENETIC)

group of related genes that determine the basic structure and orientation of an organism.

In the late 1940s, Edward Lewis began studying homeotic mutation on Drosophila melanogaster which caused bizarre rearrangements of body parts. Mutations in the genes that code for limb development can cause deformity or lead to death. For an example, mutations in the Antennapedia gene cause legs instead of the antenna to develop on the head of a fly. These mutants sometimes occur in wild populations of flies, and it was these mutants that led to the discovery of Hox genes.

HOX GENE

A cell can only be indeterminate if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is characteristic of deuterostomes - when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism.

The sea urchin is a deuterostome

INDETERMINATE CLEAVAGE

one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm (outside layer) and endoderm (inside layer), with the mesoderm as the middle layer between them.

MESODERM

Parazoans are sponges

PARAZOAN

Protostomes are animals in which the blastopore becomes the mouth

A Caribbean Reef Squid, an example of a protostome.

PROTOSTOMES

A pseudocoelomate is any invertebrate animal with a three-layered body and a pseudocoel. The coelom was apparently lost or reduced as a result of mutations in certain types of genes that affected early development. Thus, pseudocoelomates evolved from coelomates.

Roundworms are examples of pseudocoelomates

PSEUDOCOELOMATES

Radial cleavage is characteristic of the deuterostomes, which include some vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.

Sea urchins have radial cleavage

RADIAL CLEAVAGE

These organisms resemble a pie where several cutting planes produce roughly identical pieces. An organism with radial symmetry exhibits no left or right sides. They have a top and a bottom (dorsal and ventral surface) only.

RADIAL SYMMETRY

Spiral cleavage is characteristic of protostomes. A developing embryo has spiral cleavage if as it undergoes cell division (cleavage) and changes from a four-cell embryo to an eight-cell embryo, the cells divide at sli ght angles to one another, so that the none of the four cells in one plane of the eight-cell stage is directly over a cell in the other plane.

Mollusks have spiral cleavage

SPIRAL CLEAVAGE

condition of the blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. The germ layers form during gastrulation of the blastula.

Flatworms are triploblastic

TRIPLOBLASTIC