HYDROZOA 



209 



In the sea are numerous hydroid polyps which, while agreeing in the 

 main with Hydra, are distinguished from it in two important respects: 

 (i) they do not directly produce sexual organs; (2) they reproduce asexu- 

 ally, and by incomplete budding form persistent colonies. In this a series 

 of parts have arisen which recjuire special designations (fig. 171). The 

 separate animals, hydranths, are connected by a system of tubes, the cceno- 

 sarc, which, like the hydranths, consist of ectoderm, entoderm, and 

 mesoglcea, and since the gastrovascular space continues in them, these 

 distribute food throughout the colony. The ccenosarc may creep over 



e7i s ek c 



Fig. 169. Fig. 170. 



Fig. i6g. — Hydra viridis* testes above; ovarian enlargement and escaping egg 

 below. 



Fig. 170. — Body layers of Hydra (after Schulze, from Hatschek). c, cuticula; en, 

 nettle cells; ck, ectoderm; en, entoderm; s, supporting layer. 



some support (stone, alga, snail-shell, etc.) and form a network, the 

 hydrorliiza, or it may stand erect and free, forming a hydrocaidus. Usually 

 both hydrorhiza and hydrocaulus occur in the same colony. 



Usually the colony is strengtheired and protected by the perisarc, a 

 cuticrdar secretion of the ectoderm. In some (fig. 172) the perisarc stops 

 at the base of the hydranth; in others (fig. 173) it expands distally into a 

 wide-mouthed bell, the liydrotlieca, into which the hydranth may retract. 

 In rare cases this perisarc may be greatly increased and calcified, forming 

 large coral-like masses with openings from which the hydranths may 

 protrude (fig. 174). 



