IIYDROZOA. 



231 



are crowded into small ridges or batteries. The sex cells (at cer- 

 tain times) i^rodnce swellings on the column; a circle of male 

 swellings close beneath the tentacles, the female cells farther down 

 the column (fig. 172). Individuals reproducing by budding arc 

 more common than the sexually mature (fig. 90). Small eleva- 



ek 



Fig. 172. 



Fig. 172. —Hydra viridi.\* testes above; ovarian enlargement below. 

 Fig. 173. — Body layers of Hudra, (After Schulze, from Hatschek.) c, cuticula; en, 

 nettle cells; eh^ ectoderm; c?i, entoderm; .s, supporting layer. 



tions appear on the column, enlarge, form tentacles, and at last a 

 mouth, after which they may separate from the parent. 



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

 the main with Hydra, are distinguished from it in two important 

 respects: (1) they do not directly produce sexual organs; (3) they 

 rei^roduce asexually, and by incomplete budding form jiersLstent 

 colonies. In this formation of colonies a series of parts have 

 arisen which require special designations (fig. 174). The sejiarate 

 animals are the liijdranths, and are connected together by a system 

 of tubes, the cmnosarc, which, like the hydranths, consist of ecto- 

 derm, entoderm, and mesogloea, and since the gastrovascular space 

 continues in them, these effect a distribution of food throughout 

 the colony. The crenosarcal tubes may creep over some support 

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

 or it may stand erect and tree-like, forming a Inidrocaulns. Usually 

 both hydrorhiza and hydrocaulus occur in the same colony. 



