SURVEY OF INVERTEBRATES 71 



cell layers which are not homologous with the ectoderm and 

 endoderm of higher animals. All considered, Sponges probably 

 represent a side branch of the Animal Kingdom that went up, so 

 to say, an evolutionary blind alley, and remained only Sponges! 

 (Fig. 313.) 



B. COELENTERATES 



With the large group of aquatic animals comprising the Coelen- 

 terata — the Polyps, Jellyfish, Sea-anemones, and Corals — we 

 reach the basic phylum of the Invertebrates because, with a mouth 

 opening into a digestive cavity and with specialized body parts 

 coordinated by a simple nervous system, they institute the plan of 

 structure that proves to be the fruitful one for the derivation of 

 certain features exhibited in all higher animals. Our review of the 

 phylum will merely serve to indicate some of the chief forms as- 

 sumed by the polyp type of individual in the highly diversified 

 classes known as Hydrozoa, Scyphozoa, and Anthozoa. 



Hydrozoa. One of the few Coelenterates inhabiting fresh water 

 is the common polyp, Hydra. Like all polyps its body is essentially 

 a sac composed of two cell layers — ectoderm and endoderm — 

 separated by a jelly-like non-cellular layer known as the meso- 

 gloea. The opening of the sac is the mouth which leads into the 

 digestive cavity, or enteric cavity. The mouth is surrounded by 

 a circle of tentacles well supplied with nettle cells, or nemato- 

 cysts. Obviously the polyp exhibits radial symmetry — right and 

 left sides do not exist. It has no internal organs and so, of course, 

 no organ systems. The adult Hydra now and again produces on its 

 body-surface male and female sexual organs that liberate sperm 

 and eggs and, after fertilization, the zygote develops into another 

 Hydra. Furthermore, Hydra reproduces asexually by buds: small 

 outpocketings from the body wall grow into small Hydras and 

 then separate as independent polyps. (Figs. 57, 135-137, 155.) 



However, the life history of Hydra is not representative of the 

 many kinds of marine Hydrozoa that commonly grow on sub- 

 merged objects, such as the piles of piers, because most of them 

 consist of large colonies of Hydra-like individuals organically 

 connected; somewhat as though a Hydra formed many buds that 

 remained attached. Moreover, the colony is only one phase of 

 the life history of a Hydroid, for it develops special buds, known 

 as medusae, which become separated from the colony as inde- 



