116 



COMPARATIVE ANATOMY. 



of the umbrella is continued into the wall of the stomach in the 

 Discophora, the stomach is not very sharply marked off from the rest 

 of the gastric system. Its wall is always continued into arm-like 

 appendages, which, as a rule, project into folded membranes (oral 

 arms) ; the mouth is placed between these. Division of these arms 

 leads to further modifications, which give rise to greatly ramified 

 appendages. In this case numerous grooves, which gradually unite, 

 lead to the mouth, in correspondence with the form of the arms. 

 In the Khizostomidas the mouth remains open during an early period 

 of development only, and afterwards becomes closed by the gradual 

 union of the " arms," which limit it, and in which the grooves form 

 branched canals, which open at the ends of the ramifications of the 

 arms by numerous fine pores (polystomia). 



In the Lucernariae the structural conditions of the gastro- 

 vascular system closely resemble those of the Medusas. A stomachal 

 tube, projecting from the concave surface of the umbrella, and pro- 

 duced into four angles, leads into a wide space, which is continued 

 into four pouches, and may be elongated into four canals, which pass 

 into the stalk. The four pouches correspond to widened radial 

 canals, and are, as in the Medusas, connected with one another at 

 the edge of the umbrella, and so form a circular canal. In others 

 this character is modified in such a way that the stomach is continued 

 into the body, in a tubular form ; and at its end, which projects into 

 the stalk, gives rise to radial canals, which whilst becoming enlarged 



run outwards towards 

 the margin of the disc. 

 The gastrovascular 

 system in the larva? of 

 the Discophora and in 

 Scyphostoma is very 

 similar in character. 



§ 91. 



The gastric system 

 of the Anthozoa ex- 

 tends by means of an 

 oesophagus from the 

 centre of the tentacle- 

 bearing surface of the 

 body into the interior, 

 where it opens into 

 the digestive cavity. 

 From this part canals 

 pass upwards along the 

 sides of the oesopha- 

 gus into the tentacles. 

 Owing to the width of the canals connected with the stomach, 

 the intermediate tissue is reduced to a mere partition (s), which 

 extends in rays from the wall of the body to the wall of the 



Fig. 45. Transverse section through a portion of the 

 stock of Alcyoninni, in which two individuals, A A, are 

 cut through just below their junction with the ccenen- 

 chyma, and a third, B, somewhat lower, v Wall of 

 oesophagus, c Eadial canals (chambers of the body. 

 cavity), s Septa, o Ova. Part of the coenenchyma 

 traversed by canals is seen to contain calcareous bodies. 



