l82 



PHYLUM CCELENTERA 



On the convex surface of the hell 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 



Fig. 91. — Structiire of a Swimming-bell or Medusoid, 

 Obelia geniculata, budded off from a Campanularian Hydroid. 



M., Mouth on the short manubrium ; R.C., one of the four radial canals 

 from the central stomach to the circumference canal (C.C.) round the 

 margin ; G., gonad on radial canal (Leptomedusoid) ; T., numerous 

 marginal tentacles, which have small internally projecting vesicles (.S.) 

 at their base. These are not to be confused with 8 minute spherical 

 balancing organs or statocj'sts (ST.), situated adradially on the margin. 



is ciliated ; it lines the food canal, and extends also into the 

 tentacles. The mesoglcea forms a thickened jelly, present 

 more especially on the convex (ex-umbrellar) surface. 



The mouth opens into the canal of the manubrium, which 

 leads to the central cavity of the dome. With this a varying 

 number of unbranched radial canals communicate ; these 

 open into a marginal circular vessel, which communicates 



