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TEXTBOOK OF ZOOLOGY 



hydranth to form a bowllike case or hydrotheca which supports it. 

 Another modification is the taller, more enclosed case, gonotheca, 

 which nearly encloses the Mastostyle. The blastostyle with this cover- 

 ing is often called the gonangium. Fibrous processes connect the 

 perisarc to the soft, inner parts (coenosarc). The cavity of the 

 hydranth is continuous with that of the hydrocaulus, and is, there- 

 fore, a part of the gastrovascular cavity. 



Medusa e x 



J Sperm -from 



J another 



J medusa 



^__ ..Ferbilucd eqq 



\*" "^ *;> Cleavacfe 

 ^ cell stage. ^"^ 



\ 



Mature, 

 qonancjlcim.^^g 



BJastula. 



^ \arva ^ 



Position of 

 mature colony 



Fig. 59. — Life cycle of Ohelia, illustrating polymorphism and metagenesis. Adult 

 hydroid colony with mature gonangium gives rise to sexual medusa which is pro- 

 duced in the gonangium and set free in the water. Germ cells produced by the 

 medusae complete the cycle. Blastula and planula are free-swimming. (Redrawn 

 and modified from Wolcott, Animal Biology, published by McGraw-Hill Book 

 Company, Inc.) 



The coenosarc is made up of an outer layer of cells, the ectoderm, 

 just beneath the perisarc, and an inner endoderm layer lining the 

 cavity. The mouth of the hydranth is located in a domelike hypostome 

 at the free end. There are between twenty and thirty solid tentacles at- 

 tached around the basal margin of the hypostome. The hydranth cap- 

 tures and ingests small aquatic organisms as food by the aid of stinging 

 bodies (nematocysts) produced in certain ectoderm cells of the distal 

 portions of the tentacles. The digestion of this food is accomplished 



