CA3IPANIILARIAN HYDROID. 33 



2. The inner layer, or endoderm, is of about equal 

 thickness, but the cells which compose it are more opaque 

 and granular. 



3. A thin transparent supporting layer with well-defined 

 edges will be seen to separate the outer from the inner 

 layer. 



c. In the centre of the coenosarc notice the tubular 

 body cavity. 



d. In a living specimen note that the granular matter 

 which fills the cavity is kept in circulation by the action 

 of cilia which arise from the inner surface of the endo- 

 derm. 



€. [Make a sketch showing these points. 



III. Examine the hydranths, and having found one 

 which is expanded, note : — 



a. The open cup or vase, the hydrotheca (Fig. 15, a') 

 formed by the expansion of the perisarc. 



h. The crown of tentacles (Fig. 15, c) arranged in a 

 circle. 



c. The rounded prominence, or manubrium (Fig. 15, g) 

 which projects into the space between the tentacles, and 

 cari'ies the mouth upon its free end. 



d. The body (Fig. 15, h) of the hydranth, made up 

 like the stem of three lasers. 



1. The thin ti-ansparent ectoderm. 



2. A sharply-defined supporting layer. 



3. A very thin la3^er of endoderm, made up of large 

 opaque vacuolated cells, the inner or free ends of which 

 carry cilia. 



e. Notice that the body cavity of the hydranth is sim- 

 ply an enlargement of that of the hydrocaulus. It is 

 hned with cilia, and the particles of food which it usually 

 contains may in the living specimen be seen to circulate 

 throujjh the stem. 



