I 78 ZOOLOGY 



4. Nettle-cells are found in practically the whole group 

 (Fig. 83). 



5. Nerve cells (sensory) and muscle cells both occur. 



6. Reproduction by non-sexual methods is prevalent. This 

 often alternates regularly with the sexual. The individuals of 

 the two generations may be very different in appearance and 

 habits. 



7. Wholly aquatic; chiefly mnriae. 



223. General Survey. The group of Ccelenterata embraces 

 animals very diverse in general appearance, which may never- 

 theless be reduced to two types. The first and most primitive 

 is the tubular hydroid type. This is sessile and is essentially a 

 gastrula, at the unattached end of which occurs the mouth, 

 usually 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 extreme 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 



