112 ANIMAL PHYSIOLOGY. LESSON 32. 



(c) leads to the gastric portion of the intestine (d) ; and that canal 

 twice winds round the circuit of the abdominal cavity before its final 

 termination. The intestine is generally found loaded with fine sand ; 

 its surface, and that of the vascular mesentery, is covered with a rich 

 network of capillary blood-vessels. 



534. Near the oesophagus is a fusiform, dilated, contractile vesicle ; 

 this is the heart, and by tying a small pipe in it, and passing injec- 

 tion through it, the whole vascular system will be beautifully dis- 

 played. A trunk proceeds from the heart, which forms a circle round 

 the oesophagus, at the base of the lantern ; a second trunk proceeds 

 from the opposite end of the heart, and forms a similar circle round 

 the vent ; an artery and a vein also run along the concave margin of 

 the intestine ; the blood is of a yellowish color, and exhibits a dis- 

 tinct nucleus. 



HOLOTHUKIA. 



535. These animals have been likened to a lengthened Echinus, 

 deprived of its calcareous plates, and with its axis extended in a lon- 

 gitudinal direction. The skin is usually soft and leathery ; in a few 

 genera, strengthened by calcareous spines. Five avenues of suckers, 

 terminating the long retractile tubular feet, divide the body into as 

 many longitudinal segments, which, in the majority, are of equal, or 



nearly equal dimensions. In some species 



FIG. 190. 11 IT-, 



the suckers are developed only on one 

 side; in other species the body is entirely 

 covered with them ; the suckers are simi- 

 lar in every respect to those of Echini 

 and Star-fishes. 



536. The mouth is surrounded by 

 plumose tentacula, usually of great beau- 

 ty; when complete, the number is al- 

 ways Jive, or its multiple (Fig. 190). 

 The tentacula can be withdrawn into 

 the mouth by means of its proper 

 muscles, and in captivity they frequent- 

 ly tantalize the eager naturalist by re- 

 taining them in the interior of the 

 body for days together ; nay, they often 

 die with them in this position. 



537. They are provided with a circle 

 of teeth, analogous to those of the 



Cucumaria frondosa. Echini. 



