HYDROZOA. 



231 



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

 tain times) produce 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 are 

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



ek 



s ek c 



FIG. 172. 



FIG. 173. 



FIG. 172. Hydra viridis,* testes above; ovarian enlargement below. 

 FIG. 173. Body layers of Hydra. (After Schulze, from Hatschek.) c, cuticula; en, 

 nettle cells; eto, ectoderm; en, 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 polyps which, while agreeing in 

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

 respects: (1) they do not directly produce sexual organs; (2) they 

 reproduce asexually, and by incomplete budding form persistent 

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

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

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

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

 derm, entoderm, and mesogloea, and since the gastro vascular space 

 continues in them, these effect a distribution of food throughout 

 the colony. The coenosarcal tubes may creep over some support 

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

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

 both hydrorhiza and hydrocaulus occur in the same colony. 



