630 - Heredity and Evolution 



flagellated cells that become invaginated 

 when the gastral cavity begins to form. 



All in all, the evidence indicates that the 

 Porifera do not represent a main line lead- 

 ing toward the evolution of higher animals. 

 More probably they are a divergent group 

 that originated at a very early date (Fig. 

 29-11). 



THE COELENTERATA 



Probably the most familiar member of this 

 relatively small (10,000 species) primitive 

 metazoan phylum is Hydra (Fig. 3-12). How- 

 ever, the Portuguese man-of-war (Fig. 32-6), 

 the jellyfishes, sea anemones (Fig. 32-7), and 

 coral animals are also coelenterates of fairly 

 common occurrence. 



Distinguishing Characteristics of the 

 Phylum. (1) Coelenterates display an essen- 

 tially saclike structure (Fig. 16-4). Only one 

 opening, which is called the mouth, com- 

 municates between the internal cavity and 

 the outside environment. The cavity itself 

 represents a saccular type of enteron, since 

 some digestion occurs here; but also it may 

 be called a gastrovascular cavity, because it 

 tends to distribute food particles as well as 

 to digest them. Also the cavity sometimes is 

 called the coelenteron, since this one cavity 

 substitutes for the two cavities (coelom and 

 the enteron) of higher animals. 



(2) The body wall of the coelenterate con- 

 sists of only two layers of cells (Fig. 32-8). 

 However, the outer ectodermal layer is sepa- 

 rated from the inner endodermal layer by an 

 essentially noncellular matrix, called the 

 mesoglea. In Hydra, this matrix is relatively 

 thin and inconspicuous; but in many other 

 coelenterates (for example, jellyfishes) it con- 

 stitutes the bulk of the body. Such a diplo- 

 blastic organization, in which only two 

 (ectoderm and endoderm) of the primary 

 germ layers are represented, stands in distinct 

 contrast to the triploblastic organization of 

 higher animals, in which a third germ layer, 

 the mesoderm, is always represented. 



(3) The cells of the coelenterate show con- 



Fig. 32-6. The Portuguese man-of-war, Physaiia pela- 

 g/ca. Note the gas-filled saillike float, which is about 

 1 foot long and richly colored in iridescent hues of 

 purple and green. Also note the fish, ensnared in the 

 very long tentacles. Actually this coelenterate rep- 

 resents a colony of multicellular individuals: some spe- 

 cialized for stinging prey, some for feeding, and some 

 for reproduction. (New York Zoological Society Photo.) 



siderable differentiation as to form and func- 

 tion but not to the extent found in higher 

 animals. As is shown in Figure 32-8, these 

 cell types include: (a) primitive nerve cells, 

 called protoneurons; (b) gland cells; (c) epi- 

 theliomuscle cells; (d) nematoblasts (see 

 below); and (e) interstitial cells. 



(4) The different kinds of cells tend to be 



