ASTEROIDEA. 



177 



secondary and tertiary glandular diverticula, and suspended to the 

 abactinal body-wall of the arm by two mesenteries (Fig. 129, 24). The 

 two caeca of a pair usually arise separately from the pyloric sac, but 

 in some cases (Asterias, etc.) they are united near the stomach and 

 arise from it by a common tube. The proximal end of each of these 

 caeca gives off on its actinal side a pouch, called Tiedemanris pouch 

 (specially developed in the Echinasteridae and Asterinidae). The 

 abactinal side of the stomach 

 is closely applied to the body 

 wall, but between the two is 

 a variable number of inter- 

 radially placed glandular 

 diverticula : these are the 

 rectal caeca (Fig. 127, Js). 

 They vary considerably in 

 form, size and number (from 

 two to five, in Luidia they 

 are absent), and they open 

 into the rectum except in 

 the Astro pectinidae, in which 

 the rectum and anus are 

 absent and the caeca open 

 directly into the stomach. 

 The rectum is a short tube 

 which leads from the stomach 

 to the anus. The anus 

 (p. 167) is absent in the 

 Astropectinidae and Porcel- 

 lanasteridae. The alimen- 

 tary canal is lined throughout by a glandular ciliated epithelium, and, in 

 some forms at least, a few calcareous bodies are found (Culcita, Ophidi- 

 aster chinensis) both in its walls and in the mesenteries which attach it 

 to the body-wall. 



The mesenterial attachments of the alimentary canal are as follows : 

 ( 1 ) a pair of muscular strands run from the oral wall of the stomach in 

 each radius to be inserted along the ambulacral ossicles of the ray (Fig. 

 132, ret) ; they serve to retract the stomach when, it has been evaginated 

 for the prehension of food ; (2) a number of fine cords passing from the aboral 

 wall of the stomach to the body-wall (Fig. 132, mes) ; these are remains 

 of the septum between the right and left posterior coeloms of the larva ; (3) 

 the mesenteries of the hepatic caeca (p. 177 and Fig. 129, 24) ; these fuse with 

 one another distally, but proximally end freely so that the space between 

 them is in open communication with the general body-cavity. 

 They are probably remains of the mesentery which separated the right 

 and left posterior body-cavities of the larva. On this view the spaces 

 between the two mesenteries of a pair are parts of the right posterior 

 body-cavity, and, if the division between the two body-cavities were 

 retained in the adult, the two mesenteries of one caecum should join 

 respectively the mesenteries of the caeca on each side of it in the disc, so 

 as to cut off just above the stomach a circular patch of body-cavity with 

 two prolongations into each arm. (4) Two bands which pass from the 

 aboral end of the wall of the axial sinus to the stomach. These were 

 formerly supposed to be processes of the aboral sinus. 



Z III N 



FlO. 128. Asterina gibbosa with the ahactinal body- 

 wall removed (from Claus). Ld hedatic caecum 

 G gonad. 



