ACTIN0Z0A. 223 



Thus the axial system is completed. Next, radial 

 and paragastric canals appear, the former quickly 

 reaching the surface of the body, where they 

 branch and give origin to the ctenophoral canals, 

 the apical ends of which are well developed, while 

 their opposite extremities are still on their way 

 towards the oral region, in striving to gain which 

 they are outstripped by the paragastric vessels. 

 The extremities of these either remain csecal 

 (Pleurobrachia), or, by branching, give origin to 

 the two lateral rudiments of the oral canal. 

 These may still continue distinct (Bolina, Chia- 

 jea, Le Sueuria), becoming connected with the 

 extremities of the lateral ctenophoral canals ; or 

 unite to form a complete circular tube, which 

 receives, as before, only the four lateral canals 

 (Eurhamphoea), or both these and the vessels of 

 the antero-posterior ctenophores (Beroe). Thus, 

 the oral tube bears to the paragastric canals a 

 relation comparable, perhaps, with that between 

 the ctenophoral vessels and the radial system : and 

 the gradual development of the entire canal sys- 

 tem may be described as tending in a peripheral 

 direction, while its several elements bifurcate; 

 the branches so formed, to a greater or less ex- 

 tent, again uniting, and prolonging their course 

 beneath the surface of the body. 



In the above survey the canal systems of Cestum 

 and Callianira have not been included. Of the 

 latter nothing whatever is known. In Cestum the 

 axial system resembles that of Bolina and the 

 Ctenophora in general. Each half of the cteno- 

 phoral system is represented by four very long 

 canals, two of which run side by side along one of 

 the fringed margins of the ribbon-shaped body, 



