476 COMPARATIVE ANATOMY CHAP, vin 



'2. The alimentary canal runs direct, without coiling, from mouth 

 to anus, but has sac-like widening^ (Opkiuroidea), and further, in the 

 Asteroids, sends off branched outgrowths into the arms. 



The wall of the intestine, as a rule, in the Echinoderm, consists of 

 the following layers : (1) a deep inner epithelium, with numerous 

 glandular cells ; (2) a layer of connective tissue ; (3) a muscle layer ; 

 (4) an outer epithelium, the endothelium of the body cavity. A 

 system of blood lacunae (absorbing canals) is developed in the layer 

 of connective tissue in the J/alofhttrin/i/^i, Kc1iliin'nl<-n, and 1'i-hmidea. 



B. Holothurioidea (Figs. 371, p. 451, and 383). 



Course of the alimentary canal. The mouth lies at the oral 

 pole (i.e. at the anterior end of the body), the anus at the apical pole. 

 For the exceptions to this rule, especially Ji'lin}>ii/i/iinf, in which the 

 mouth lies close to the anus, rf. section III , p. 408. 



The alimentary canal is, as a rule, considerably longer than the 

 body (on an average three times as long), and therefore has a looped 

 or winding course. From the mouth, it first runs backward towards 

 the anus (first or anterior section), it then bends for the first time, 

 and runs forward again (second or middle section) ; lastly, it bends 

 again near the anterior end of the body, and runs backward once 

 more, this time to the anus (third or posterior section). 



In making these bends, the alimentary canal forms a spiral round 

 the principal (longitudinal) axis of the body ; this is very clearly 

 seen by following the line of attachment of its mesenteries to the 

 body wall. 



The anterior section is attached to the median dorsal line interradially. From 

 this, at the first bend, the mesentery passes across the left dorsal radius into the left 

 dorsal interradius. The whole of the central section is attached in this interradius. 

 At the second bend, the mesentery passes over the left ventral radius and interradius, 

 and over the middle ventral radius, into the right ventral interradius. The third 

 or posterior section is attached in this latter interradius (Figs. 350, p. 407, and 

 383). 



If a Holothurian is placed upright, with the oral pole upward and 

 the apical downward, and if we project the loops of the alimentary 

 canal on to a horizontal plane, or, if we simply view the intestinal 

 loops of a Holothurian from the oral pole, we see that the digestive 

 tract runs from left to right, i.e. in the direction of the hands of a 

 clock. In other Echinoderms with coiled intestine, the coils also run 

 in this direction. 



It was mentioned above that the alimentary canal of many Parac- 

 tinopoda (Synaptidas) is almost straight. This is, however, not an 

 original condition, as is seen from the following facts : (1) the older 

 larva and the quite young S>tnnpf<t have a bent alimentary canal ; (2) 

 the intestinal mesentery is inserted in the body wall exactly in the 



