84 THE ECHINODERMATA. $$ 82, 83. 
CHAPTER V. 
DIGESTIVE APPARATUS. 2 
§ 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 
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 Crinoidea, 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 Hchinoidea, 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 cavity of the body. These tentacular vesicles contain a liquid, 
and, in the Asteroidae, where they are upon the 
ventral surface at the end of the furrows, the rays 
bend round to the dorsal surface; and again, 
although Tiedemann ‘Meckel’s Deutsch. Arch, 
loc. cit. p. 175) thinks these last can distinguish 
light from darkness, yet it is doubtful if these ani- 
mals can really see by these organs. They appear, 
like many other inferior animals, to perceive the 
light by its action as an excitant upon their skin, 
and in this way can, like plants, seek the sun- 
light. The account which Forbes (Hist. of British 
Star-fishes, p. 139, and Froriep’s neue Not. No. 
420, 1841, p. 26) has given of Luidia fragilis- 
sima, which, having made its escape by the loss 
of an arm, looked with scornful eyes upon its per- 
secutor, is pleasant to read, but is far from set- 
tling this question. 
1 Asteroidea, Eck d 
, and Sip loidea. 
2 Holothurioidea, 
8 Holothurioidea and Sipunculoidea. 
4 Asteroidae. 
5 Ophiuridae. 
6 According to Sharpey (Cyclopedia, &c., loc. 
cit. I. p. 616) and Valentin (Wagner’s Hand- 
worterbuch der Physiol. I. 1842, p. 493), the inter- 
nal surface of the st h and its appendages, of 
the Asteroidae, has a ciliary movement. Valentin 
(Monogr. &c. p. 79) has also found ciliated epithe- 
lium in the entire digestive canal of Echinus. 
With Phascolosoma, where I have found cilia 
upon the tentacular apparatus, and with Coma- 
tula, where Miller (Abhandl. d. Berl. Akad. 
1841, p. 233) has found them in the anus, they 
extend probably through the intestine. 
1 Miller, Abhandl. d. Berl. Akad. 1841, p. 222. 
