84 THE ECHINODERMATA. ^'J» 82, 83. 



CHAPTER V. 



DIGESTIVE APPARATUS. 



§ S2. 



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 

 Holothurioidea, 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 Crinoi'dea, it is near the mouth upon the ventral surftice, and 

 in the Sipunculoi'dea, 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 perhaps 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 cavit}' of the body. These tentacular vesicles contain a liquid,. 



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



ventral surface at the end of the furrows, the rays 3 Holothurioidea and Sipunculoi'dea. 



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



although Tieilemann [Meckel's Deutsch. Arch. 6 Ophiuridae. 



loc. cit. p. 175) thinks these last can distinguish 6 According to Sharpey (Cyclopaedia, &c., loc. 

 light from darkness, yet it is doubtful if these ani- cit. I. p. 616) and yalenim (fVaf^ner's Iland- 

 mals can really see hy these organs. They appear, wiirterbuch der Physiol. 1. 1842, p. 493), the inter- 

 like many otlier inferior animals, to perceive the nal surface of the stomach an<i its appendages, of 

 light by its action as iin 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\i neue Not. No. With J'hasco/osoma, where I liave found cilia 

 420, 1841, p. 26) has given of Luidia fragilis- upon the tentacular ai>paratns, and with Coma- 

 sima, which, having made its escape by the loss tula, where Müller (Al)handl. d. Berl. Akad. 

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

 «ecutor, is pleasant to read, but is far fi-om set- extend jirolialily through the intestine. 

 tling this (juestion. 1 MuHer, Abhaudl. d. Berl. Akad. 1841, p. 222, 

 1 Asteroidea, EchinoHdea, and Sipunculoldea. 



