70 HOLOTHURIANS 



The Heart Urchins (Spatangoidea) have lost the ." Aristotle's 

 Lantern " altogether, the only trace of its former presence being 

 a canal with membranous walls encircling the mouth, which has 

 the form of a transverse slit, is situated near the front end of the 

 test, and has the posterior lip projecting considerably forward. 

 The anus is marginal or on the under surface of the test. In the 

 Common Heart Urchin (Echinocardium cor datum) there are four 

 distinct types of tube-feet, namely : (i) Respiratory tube-feet of 

 the rays or petals of the upper surface of the test, which have no 

 terminal sucker, and broad flat bases. (2) Prehensile tube-feet of 

 the anterior ambulacrum, which when fully expanded are several 

 times longer than the body of the Heart Urchin, and have curious 

 frayed edges to their disks. (3) Buccal tube-feet, which are short, 

 thick, pointed, and covered with numerous club-shaped pro- 

 cesses. (4) Degenerate tube-feet, which are single, few in num- 

 ber, and issue from single pores of the test. The Heart Urchin is 

 brown in colour, about 2 to 3 inches long, of an oblong oval shape, 

 covered with fine spines. It forms a burrow in the sand some 

 8 to 10 inches below the surface, with which it communicates by 

 a narrow, cylindrical opening. On wide, flat, sandy shores these 

 curious Urchins may sometimes be seen emerging from their safe 

 retreat near to low-tide mark, and making their way towards the 

 incoming waves. 



The fourth class of the Echinozoa contains the Holothuroidea, 

 the Holothurians or Sea-cucumbers, all of which are creatures with 

 more or less rounded, elongated bodies, having the mouth at one 

 end and the anus at the other ; while the ambulacral grooves, repre- 

 sented by closed canals, run from near the mouth down the length 

 of the body to the proximity of the anus. The sausage-shaped 

 body is enclosed in a tough skin, and the skeleton is reduced to 

 more or less scattered spicules or plates, often very beautiful and 

 regular in form. The gullet leads into a long, coiled digestive tube, 

 which terminates in a large pouch or cloaca at or near the opposite 

 extremity of the body. Round the mouth there is a fringe of 

 branched tentacles connected with the water-vascular ring. In 

 most of the Holothurians the water-tube which connects with 

 the water-vascular ring hangs down freely in the body cavity and 

 terminates in a sieve-like madreporite ; but in a few species this 

 is not the case, the water-vascular ring communicating directly 



