84 COMPARATIVE ANATOMY CHAP. 



divisions being superimposed one on another in the principal axis. 

 The lowest division is the opal stomach, continued in the oral or 

 gastric peduncle, which hangs down from the middle of the sub- 

 umbrella. The gastric peduncle, at whose free end lies the mouth, may 

 be very variously developed, from a short wide tube to a long tubular 

 structure protruding far beyond the subumbrellar cavity. The mouth 

 is either simple, square, or cross-shaped, or produced into 4 points 

 or lobes, and sometimes supplied with oral tentacles, or with variously 

 shaped papillae. The edge of the mouth is generally well armed with 

 nemato-cysts. 



The gastro-canal system of the Acraspeda or Scyphomedusce 

 (Fig. 67, p. 77 ; Fig. 70) shows, in the arrangement of its single 

 sections, a still more varied structure than that of the Craspedota. In 

 some groups it is wonderfully complicated, and sometimes shows great 

 similarity with the gastro-canal system of certain Craspedota ; in such 

 cases, when we further think of the great similarity in body form, it is 

 difficult to believe that the Acraspeda and Craspedota are two sharply 

 divided branches of the Cnidaria. 



Let us first consider the oral tube or oral peduncle, which, unlike 

 that of the Craspedota, contains the ectodermal oasophagus. The oral 

 tube is usually short, and has either a simple square or cross-shaped 

 aperture, or, as in most large Acraspeda, is produced into 4 long 

 strong oral arms. These 4 perradial oral arms become, by means 

 of a bisection so deep as to reach their bases, the 8 oral arms of the 

 PJiizostomce, which are distinguished by the following peculiar organisa- 

 tion. 



Each oral arm becomes deeply furrowed on the side turned to the 

 chief axis, forming a channel in the longitudinal direction. This 

 inner channel corresponds with an externally projecting mid-rib. 

 The channel becomes deeper, and the curled edges of the oral arm 

 which border it unite over it and grow together, so that it now 

 becomes a closed canal. Such concrescence is completed along the 

 whole length of the oral arm to its base, and also spreads to the edges 

 of the oral aperture lying at the base of the arms, and the canal thus 

 becomes completely closed. The concrescence of the curled edges of 

 the arm, however, takes place in such a way that numerous small open 

 canals remain (suctorial mouths) (Fig. 70 D, sin) these lead from the 

 exterior into the central canal of the arm. This again leads into the 

 closed oBsophagus. In all Rliizostomce the original oral aperture is thus 

 closed, and is replaced by the numerous suctorial mouths on the hollow 

 oral arms. 



In the endodermal gastro-canal system, here as among the 

 Craspedota, we can distinguish a central or main intestine from the 

 peripheral intestine (Kranzdarm). The main intestine can separate 

 into two divisions, one lying above the other, the lower of which 

 always communicates with the peripheral intestine. 



On the wall of the main intestine of all Acraspeda (as opposed to 



