Introductio7i to Animal Morphology. 85 



make up the hydrophj^on. The wall of the tubular 

 somatic cavity is composed of endo- and ecto-derm, 

 covered by a chitinous, unorganized sheath (periderm), 

 a differentiation or secretion from the ectoderm. In 

 Hydra, the zooids which produce the sexual products 

 are similar to those which do not ; but in these orders 

 the nutritive and generative zooids are dissimilar. 

 All the nutritive zooids of a colony, taken together, 

 constitute the trophosome ; all the generative, the 

 gonoso77ie ; either set may be homomorphic (similar) or 

 heteromorphic (dissimilar). Each zooid of the tropho- 

 some is somewhat like a Hydra, and is called a 

 hydranth (or polypite),* the distal end of which is 

 prolonged into a proboscis (hypostome), at whose ex- 

 tremity is the mouth, and around whose base are the 

 tentacles, often irregularly scattered, and varying from 

 2 (Lar) to 100 or more. The cavities of the tentacles 

 are either divided by horizontal partitions (septate), or 

 else obliterated (Coryne). The endoderm and ecto- 

 derm are separated by a thin, hyaline, supporting 

 membrane, lying on the endodermal side of the 

 muscular layer [Schulze]. The endoderm is ciliated in 

 many forms. 



If the hydrocaulus be cut in a living colony, the 

 distal end of the proximal segment develops a 

 hydranth, while the cut end of the distal portion only 

 forms an extension of the coenosarc. 



The gonosome is more variable than the tropho- 

 some, and consists of modified hydranths budding 

 fi-om the coenosarc ; these may be sessile closed sacs 

 (sporosacs), consisting of a process of the ectoderm and 



* Hydranths of some Tubularias become free, but only as a first stage 

 of decadence ; they never undergo any after metamorphoses. 



