84 THE ECHINODERMATA. ^"§> 82, 83. 



CHAPTER V. 



DIGESTIVE APPARATUS. 



§82. 



The alimentary canal is situated in the cavity of the body, isolated, but 

 is retained in its place by a kind of mesentery which is composed of 

 fibres,'^' or of a thin membrane.*-' 



The mouth, which is usually central, is often surrounded by a circle of 

 tentacles.*'' In the Asteroidea, the digestive canal is a large central pouch, 

 an anus and appendages extending into the rays being present in some**^ 

 and wanting in others.*^' In the other Echinoderms, the digestive canal has 

 usually thin walls, is of a variable length, and tortuous quite to the anus. 



The position of the anus is quite varied. In the Echinidae, and Aste- 

 roidae, it is in the centre of the back, exactly opposite the mouth. In the 

 Holothuvioidca, it is at the posterior end of the body ; while in the 

 Clypeastridae, and Spatangidae, it opens laterally upon the margins of the 

 shell. In the Crinoidca, it is near the mouth upon the ventral surface, and 

 in the Sipunculoidea, it has a similar position. 



The internal surface of this canal has generally been found lined with 

 ciliated epithelium.*"' 



§83. 



With the Asteroidea, and Echinoidea, the pedicellariae already described, 

 are used to seize the food and convey it to the mouth. Their ambulacra 

 are pei-haps sometimes used in the same way. In the Crinoidea, the fur- 

 row of the tentacles, aided by the tentacles themselves, serves well to con- 

 duct the food from the arms and pinnulae to the mouth. *^' 



In the Holothurioidea, and Sipunculidae, there are completely retractile 

 tentacles of a special nature. In the first, they are hollow, pinnated or 

 branched, and, arranged in a circle around the mouth, are attached by 

 their base to the osseous circle and to the elongated vesicles which project 

 into the cavity of the body. These tentacular vesicles contain a liquid, 



and, in the Asteroidae, where they are upon the 2 Ilolothurioidea. 



ventral surface at tlie end of the furrows, the rays 3 Ilolothurioidea and Sipunculoidea. 



bend round to the dorsal surface ; and again, 4 Asteroidae. 



although Tiedemann (MeckePs Deutsch. Arch. 5 Ophiuridae. 



loc. cit. p. 175) thinks these last can distinguish 6 According to Sharpey (Cyclopa;dia, kc, loc. 

 light from darkness, yet it is doubtful if these aui- cit. I. p. 616) and Valentin (lf^a,s;ncr\<i Iland- 

 mals can really see by these organs. They appear, wiirterbuch der Physiol. I. 1842, p. 493), the inter- 

 like many other inferior animals, to perceive the nal surface of the stomach and its appendages, of 

 light by its action as an excitant upon their skin, the Asteroidae, has a ciliary movement. Valentin 

 and in this way can, like plants, seek the sun- (Monogr. &c. p. 79) has also found ciliated epithe- 

 light. The account which Forbes (Hist, of British lium in the entire digestive canal of Echinus. 

 Star-fishes, p. 139, and Froriep^s neue Not. No. With Phascolosoma, where I have found cilia 

 420, 1841, p. 26) has given of Luidia fragilis- upon the tentacular apparatus, and witli Coma- 

 sima, which, having made its escape by the loss tula, where Mailer (Abhandl. d. Berl. Akad. 

 of an arm, looked with scornful eyes upon its per- 1841, p. 233) has found them in the anus, they 

 secutor, is pleasant to read, but is far from set- extend probably through the intestine. 

 tUng this question. 1 MuUer, Abhandl. d. Berl. Akad. 1841, p. 222. 

 1 Asteroidea, Echinoidea, and Sipunculoidea. 



