150 PHYLUM C@LENTERA. 
The medusoid type is like an inflated hydroid adapted 
for swimming. It is bell-shaped, and down the middle of 
the bell hangs a prolongation—the manubrium—which 
terminates in the mouth. Around the margin of the bell 
there is a little shelf, the velum or craspedon, which projects 
inwards, and is furnished with muscle cells. The margin of 
Fic. 71.—Bougainvillea.—After Allman. 
A. Asmall piece of a hydroid colony. 
p-, Perisarc ; #., medusoid bud; 4., hydranth or polyp head. 
B. A medusoid ; #a., manubrium; ~.c., radial canal; s., sense- 
organ. 
the bell also bears tentacles, usually hollow, and abundantly 
furnished with stinging cells (Fig. 65, 3). 
On the convex surface of the bell the ectoderm forms 
simply an epithelial layer; on the concave surface it is 
differentiated into muscle cells on the velum, the manu- 
brium, and the tentacles, nerve cells at the base of the 
velum, and stinging cells on the tentacles. The endoderm 
is ciliated ; it lines the food canal, and extends also into the 
