from all other types with which they are united only in having 

 a common ancestor in the Protozoa. 



In the cœlenterates the embryo very generally is a planula, 

 consisting of an outer layer of ciliated ectoderm and an inner 

 hollow mass of endoderm, and distantly comparable to a 

 phanerogam embryo. Development, which is direct and 

 immediate as in the sponges, is radially about the original 

 axis of the embryo as a center into a radially symmetrical 

 animal with a single cavity in the body. 



The cœlenterates represent the most primitive type of true 

 multicellular animal in that the polyps (or medusa) are 

 definite and complete entities, and at the same time are radially 

 symmetrical. The compactness and marked tendency toward 

 a spherical or radial form or grouping which is the chief 

 characteristic of the Protozoa naturally and logically finds its 

 first multicellular expression, as a result of the geometrical 

 division of the ovum into 2, 4, 8, 16, 32, 64, etc. cells, in the 

 formation of a spherical or radially symmetrical multicellular 

 embryo which may be a solid mass or a hollow ball of cells, 

 usually becoming flattened or more or less concave on one 

 side. Developing along the same lines by which it originated 

 from its protozoan ancestors this embryo gives rise to a 

 radially symmetrical animal with a central cavity, the axis 

 of the radial symmetry being the same as the axis of the 

 embryo. 



So far as their form, considered purely as the continued 

 and developed expression of the difference between animals and 

 plants as seen in the unicellular types, is concerned the 

 cœlenterates are the most perfect of all animals. Through the 

 formation of a multicellular body they have overcome the limi- 

 tations in size imposed by a unicellular body; but a multicellular 

 body formed by development strictly along radial lines is 

 necessarily radially symmetrical, and radial symmetry means 

 the absence of a head end and of centralized nervous control, 

 and hence of the pov/er of consciously directed locomotion. 



In the plants an asexual generation is always present ; there 

 may be a regular alternation of sexual and asexual generations 

 with the latter so reduced as to be almost vestigial as in the 

 phanerogams, there may be several or many asexual generations 

 intervening between the sexual generations, or the sexual 



(400) 



