THE COELENTERATA 



253 



various places, and is expanded into gonothecae and cup-shaped hydro- 

 thecae ( ) to accommodate the hydranths. 



A shell extends across the base of the hydrotheca which serves to 

 support the hydranth. The soft parts of the hydrocaulus and of the 

 stalks of the. hydranths constitute- the coenosarc ( ) 



and are attached to the perisarc by minute projections. The coenosarcal 

 cavities of the hydrocaulus open into those of the branches and thence 

 into the hydranths, producing in this way a common gastro-vascular 

 cavity. 



The coenosarc consists of two layers of cells an outer layer, the 

 ectoderm, and an inner layer, the entoderm. These layers are continued 

 into the hydranth. The mouth 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 and consists of an outer layer 

 of ectodermal cells and a single axial row of entodermal cells with a 

 large number of nematocysts at the extremity. The hydranth captures, 

 ingests, and digests food just as does Hydra. 



The reproductive organs develop quite like the hydranths, as buds 

 from the hydrocaulus. They thus represent modified hydranths. The 

 central axis of each is called a blastostyle ( ), and 



together with the gonothecal covering, is known as the gonangium 

 ( ). The blastostyle gives rise to medusa-buds 



which soon become detached and pass out of the gonotheca through the 

 opening in the distal end. 



Some medusae produce eggs, and others, sperm. The fertilized eggs 

 again develop into colonies like those 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 medusae of Obelia are shaped like an umbrella 

 with a fringe of tentacles and a number of organs 

 of equilibrium on the edge. Hanging down from 

 the center is the manubrium 

 ( ), with the 



mouth at the end. The gastrovas- 

 cular cavity extends out from the 

 cavity of the manubrium into four 

 radial canals on which are situated 

 the reproductive organs. 



"The germ-cells of the medusae 

 of Obelia arise in the ectoderm of 



A. 



B. 



Fig. 157. 

 A. Liriope Exigua (family Geryoniidae) . 



(Mayer). 



B. Hydraiike stage in the development of the manubrium, and then migrate 



along the radial canals to the repro- 

 ductive organs. When mature, they 



Gonionemus. One of the tentacles is carrying 

 a worm (w) to the mouth. Tentacles in con- 

 tracted state. (From the Cambridge Natural 

 History, after Perkins). 



