CCELENTERATA 1 79 



4. Nettle cells in practically the whole group (Fig. 83). 



5. Nerve cells (sensory) and muscle cells present. 



6. Reproduction commonly by non-sexual methods, often 

 alternating regularly with the sexual. Individuals of the 

 two generations sometimes very different in appearance and 

 habits. 



7. WhoUy aquatic; chiefly marine. 



223. General Survey. — The Coelenterata embrace animals 

 very diverse in general appearance, which may nevertheless 

 be reduced to two types. The first and most primitive is the 

 tubular hydroid type. This is sessile and is essentially a gas- 

 trula, at the unattached end of which occurs the mouth, usu- 

 ally surrounded by tentacles. The cavity of the tentacles is 

 continuous with the gastro- vascular cavity (Fig. 81). Of this 

 type we may distinguish two conditions: (i) in which the in- 

 dividuals (polyps) occur singly {Hydra), or if in colonies, the 

 various individuals have the same form (as the corals); (2) 

 colonial forms in which the individuals making up the colony 

 are very different (as the Siphonophora) , embracing open- 

 mouthed nutritive individuals, mouthless reproductive polyps, 

 protective polyps abundantly supplied with nettle-cells, bladder- 

 like supporting polyps, etc. (Figs. 86, 87). The extrerne con- 

 ditions of (i) and (2) are connected by forms possessing inter- 

 mediate degrees of polymorphism. Though the individual 

 polyps are attached, the whole colony may float freely. The 

 second type is the active jelly-fish, or medusoid (bell) type. 

 The medusae,, though varying greatly as to details, agree in 

 having a shape comparable to that of an umbrella or a bell 

 (Fig. 82, 6). The convex surface is normally the upper surface. 

 At the margin? of the umbrella are tentacles — often very numer- 

 ous, and frequently much elongated. In the middle of the 

 concave surface is a projection, at the lower end of which is the 

 mouth-opening. The gullet leads from the mouth into a cavity 

 in the central portion of the body of the bell (gastro-vascular 

 cavity). From the central cavity radiating passages run 

 through the substance of the bell to the margin where they 

 may communicate with a circular canal which passes around the 



