PHYLUM COELENTERATA 87 



which the colony is attached to solid objects, the main stem, or hydrocaulus, 

 and the branches, each one of which terminates in an individual which bears a 

 general resemblance to Hydra, and is called a zooid, hydranth, or polyp. The 

 whole colony is spoken of as a hydroid colony. The zooids are produced by 

 budding from the stems, just as in Hydra. In fact if the buds of Hydra should 

 remain attached to the parent a hydroid colony would be produced. Make a 

 small sketch to show the general appearance of the colony. 



2. Detailed structure. Mount some of the preserved material on a slide 

 or examine the slide of a hydroid in your box. Use the low power. The hydro- 

 caulus and branches are covered by a horny layer, the perisarc, which they secrete. 

 The perisarc extends up around each of the zooids in the form of a wineglass, 

 the hydrotheca. At the base of each branch the perisarc is ringed, probably to 

 make the stem more flexible. Within the perisarc is the coenosarc, a hollow tube 

 comparable to the column of Hydra and consisting, like the latter, of ectoderm, 

 endoderm, and mesogloea. Its cavity is the gastrovascular cavity and is con- 

 tinuous throughout the colony. There are two kinds of zooids. 



a) Nutritive zooids: This is the more abundant kind of zooid. It consists 

 of a cylindrical body from which arises the club-shaped manubrium, terminating 

 in the wide mouth opening. At the base of the manubrium is a circlet of tentacles 

 bearing nematocysts. The hydrotheca has a sort of shelftike extension inward 

 at the point where the body of the zooid continues into the coenosarc. Draw a 

 nutritive zooid. 



b) Gonozooids: These modified zooids will be found where the branches 

 arise from the hydrocaulus. Each is surrounded by a cylindrical case of perisarc, 

 called the gonotheca, open at the end. The gonozooid consists of a central stalk 

 or blastostyle, which is a degenerate nutritive zooid, and a number of saucer- 

 shaped bodies borne upon the blastostyle and almost concealing it from view. 

 The saucer-shaped bodies are medusa buds. They are formed by a process of 

 budding from the blastostyle, and eventually they become free, escape from the 

 gonotheca, and swim about in the water as little bell-shaped animals, called 

 medusae. The medusa is the sexual individual and gives rise to eggs or sperms 

 (on separate medusae) ; the fertilized egg develops into a new colony. Have the 

 assistant show you a medusa. Draw a gonozooid. 



In these colonial coelenterates there exist then three kinds of individuals: 

 the nutritive zooid, whose function is that of food getting; the gonozooid, whose 

 function is to bud off medusae; and the medusa, whose function is reproduction. 

 Such a condition is known as polymorphism, or division of labor, because the 

 organism instead of differentiating into organs for the performance of different 

 functions, as the frog does (which its simple structure does not indeed permit) , 

 differentiates into several kinds of individuals for the performance of various 

 functions. The hydroids also illustrate the principle of metagenesis, or alterna- 

 tion of generations (Hegner, p. 141), that is, the medusa or sexual generation is 



