EYDROZOA. 4 1 



or spermaria, and enclosing a diverticulum of the 

 somatic cavity (/i^. 6, a). The proximal ex- 

 tremity of this and other kinds of gonophore 

 usually becomes narrowed into a short stalk of 

 attachment. Such a form of the reproductive 

 bud is of comparatively rare occurrence. Ex- 

 ampleiS may, however, be found in some species of 

 Hydractinia, Covyne, and Clava. 



To understand the structural modifications of 

 the gonophores in other Hydrozoa, it is necessary 

 to trace briefly the principal phenomena which 

 the more complex of these bodies present in the 

 course of their development. 



All gonophores first appear as simple processes 

 of some portion of the body-wall, with its two 

 layers, the ectoderm and endoderm. Next, the 

 process becomes better defined, and exhibits a 

 peduncle or stalk. '^ When this process has at- 

 tained a certain size, its distal wall becomes 

 thickened, and projects as a sort of rounded boss 

 into the cavity, which, in consequence, becomes 

 cup-shaped. As the process enlarges, the upper 

 part of the cup-shaped cavity extends between the 

 rounded central boss and the outer wall, under the 

 form of four canals, which run up parallel with 

 the axis of the process, but stop short of its ex- 

 tremity. Their csecal ends then send out lateral 

 processes, so as to become "f shaped, and ulti- 

 mately the lateral processes unite together, so as 

 to give rise to a circular canal uniting the ends of 

 the four longitudinal canals. Contemporaneously 

 with these changes, the axis of the boss becomes 

 hollowed out by a canal continuous with the 

 original cavity of the process, and like it lined by 

 the endoderm. A separation now takes place be- 



