150 



PHYLUM CCELENTERA. 



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 



I 



FIG. 71. Bougainvillea. After. Allman. 



A. A small piece of a hydroid colony. 



/., Perisarc ; m., medusoid bud ; A., hydranth or polyp head. 



B. A medusoid ; ma. t manubrium ; r.c. t radial canal ; j., 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 



