80 COMPARATIVE ANATOMY CHAP. 



the Medusae (or the radial gastric pouches of the Corals}, branch out 

 from the stomach in various directions through the jelly, reaching 

 almost to the surface of the body. We can distinguish four kinds of 

 these canals. 



A. Four canals which to a certain extent rise interradially out of 

 the stomach. Each of these canals branches dichotomously, and 8 

 adradial canals thus arise and run to the ribs, where they enter as 

 many adradial meridian or rib vessels, which run under the ribs. 



B. Two canals which run along the broad side of the oesophagus 

 perradially and in the lateral plane (i.e. in the lateral perradii) 

 towards the oral pole, where they end blindly; these are the ceso- 

 phageal vessels (" gastric vessels " of authors) (sg). 



C. Two canals which arise perradially and in the lateral plane 

 (i.e. in the lateral perradii), and run to the walls of the tentacle sacs 

 (tentacular vessels) (tg). 



D. An unpaired canal (ag) running in the chief axis of the body 

 towards the aboral pole, and dividing, under the sensory body into 

 2 branches, which lie in the median perradii. Each of these branches 

 again divides into 2, and so 4 interradial small branches arise ; 

 2 of these, which are diametrically opposite, generally open exter- 

 nally at the sensory pole, while the others, at right angles to the latter, 

 end blindly. Less frequently all 4 branches open externally. The 

 sensory body lies between these 4 branches of the aboral vessel 

 (" funnel vessels " of authors). 



In many Ctenophora there is a further development of two solid 

 pinnate tentacles, into which the gastro-canal system is not continued. 

 In quite young animals the tentacles lie near the aboral pole. At a 

 later stage, however, they move towards the oral pole. 



The tentacles are inserted at the base of sac-like depressions of the 

 outer integument, the so-called tentacle sheaths (ts) into which they 

 can be withdrawn. They lie in the lateral perradii. In many Cteno- 

 phora the body is round in transverse section ; in others it is com- 

 pressed either in the lateral or the median plane. The lateral com- 

 pression is so great in the Cestidce that the body appears drawn out 

 into a long ribbon in the median plane. 



If we now compare the Ctenophoral forms with other Cnidarian forms, we are at 

 once struck by important distinctions between them. The Ctenophora, depart in a 

 peculiar manner, though in a direction different from that of the Corals, from the 

 purely radiate fundamental form, the two cross axes being unequal. They agree 

 with the Scyphozoa in the possession of an ectodermal oesophagus, but otherwise 

 differ decidedly from them. 



According to the opinion of some observers, the Ctenophora must be derived from 

 Craspedote Medusae. The transition from the one group to the other is made evident 

 by an interesting Cladonemid, Ctenaria Ctenophora. In this animal the umbrella 

 is much vaulted, the subumbrella much deepened ; 4 radial canals rise out of the 

 stomach and bifurcate into 8 adradial canals. There are only two pinnate, per- 

 radially placed tentacles. We need only say that the subumbrellar cavity of Ctenaria 



