COMPARATIVE ANATOMY 



for digestion; some become protective scales, some are reproductive, 

 some become thirty-foot tentacles armed with powerful nettle cells for 

 defense. Such a colony may contain a thousand individuals, whose differ- 

 ent kinds simulate the differentiated organs of higher forms and help to 

 explain how such organs arose. 



Each polyp, jellyfish, sea-anemone, or coral, like one of the sponges, 

 possesses a central cavity, the enteron. The body wall is at least two- 

 layered, with an ectoderm on the outside and an endoderm lining the 



C. SECT - BODY-WALL A. HYDRA- LONGIT. SECTION B. CROSS SECTION 



Fig. 4. — Hydra is a typical genus of coelenterates. The body-wall consists of two 

 layers with a suggestion of an intermediate mesogloea. The single body cavity is 

 the enteron or digestive cavity. 



enteron. In all except such simple forms as hydra, there is a thick 

 mesogloea which makes up the greater part of the body mass. 



Like the sponges, the coelenterates have an enteron, no longer a mere 

 passage for water, from which individual cells pick up food, each for itself, 

 but a digestive organ — hence the name coelenterate. The single opening 

 into the enteron is both mouth and anus. Muscles, also, are more differ- 

 entiated; so that the polyp waves its tentacles vigorously about, seizes 

 food and conveys it to the mouth, or withdraws suddenly into its capsule. 

 The jellyfishes swim slowly by opening and closing the umbrella. There 

 is also a simple nervous system; and in the jellyfishes, sense organs, both 

 eyes and organs of equihbrium, are arranged along the edge of the 

 umbrella. 



The symmetry of the body is generally radial, usually on four or six 

 axes, not on five as in the echinoderms. This radial symmetry may 



