PHYLUM CCELENTERATA 121 



an outer layer, the ectoderm, and an inner layer, the entoderm. 

 These layers are continued into the hydranth (Fig. 73, A, i and 

 2). The mouth (3) is situated in the center of the large knob- 

 like hypostome, and the tentacles, about thirty in number, are 

 arranged around the base of the hypostome in a single circle. 

 Each tentacle is solid, consisting of an outer layer of ectoderm 

 cells (/) and a single axial row of entoderm cells; at the extrem- 

 ity are a large number of nematocysts. The hydranth captures, 

 ingests, and digests food as in Hydra. 



The reproductive members arise, as do the hydranths, as buds 

 from the hydrocaulus, and represent modified hydranths (Fig. 

 73, 8, g, 10). The central axis of each is called a blastostyle (8), 

 and together with the gonothecal covering is known as the 

 gonangium. The blastostyle gives rise to medusa-buds (Fig. 73, 

 9) which soon become detached (Fig. 73, B) and pass out of 

 the gonotheca through the opening in the distal end. 



Some of the medusa; of Obelia (Fig. 73, B) produce eggs, and 

 others produce spermatozoa. The fertilized eggs develop into 

 colonies like that which gave rise to the medusae. The medusae 

 provide for the dispersal of the species, since they swim about 

 in the water and establish colonies in new habitats. The 

 structure of a medusa (Gonionemus) will be described in sec- 

 tion c of this chapter. The medusa of Obelia is shown in Figure 

 73, B; it is shaped like an umbrella with a fringe of tentacles 

 (2) and a number of organs of equilibrium (5) on the edge. 

 Flanging down from the center is the manubrium (1) with the 

 mouth at the end. The gastrovascular cavity extends out from 

 the cavity of the manubrium into four radial canals (4) on which 

 are situated the reproductive organs (3). 



The germ-cells of the medusae of Obelia arise in the ectoderm 

 of the manubrium, and then migrate along the radial canals to 

 the reproductive organs. When mature, they break out into 

 the water. The eggs are fertilized by spermatozoa which have 

 escaped from other medusae. Cleavage is similar to that of 

 Hydra, and a hollow blastula and solid gastrula-like structure are 



