552 



ECHINODERMATA ECHINOIDEA 



CHAP. 



when the habits of the animal are known. The function of the 

 respiratory tube-feet requires, of course, no special elucidation, 

 but the peculiar anterior ambulacrum was a mystery till the 

 feeding habits of the animal were observed by the late 

 Dr. Eobertson^ of Cumbrae. He found that the animal 

 protruded the long prehensile tube-feet through the opening 

 of the burrow up to the surface of the sand. "With their 

 iinger-like processes they then collected the surface film of the 

 sand, which was impregnated with Diatoms and other small 



Fig. 246. — Interior of test of Semtaster Fig. 247. — Dissection of Schinocardium cor- 



philijjpi, sliowing the genital organs 

 and their ducts (only three are de- 

 veloped in this species). Slightly 

 enlarged. (From Wyville Thomson. ) 



datum. X 1. The oral part of the test has 

 been removed, caec, Blind pouch of the 

 stomach ; gon, genital organ ; int, intes- 

 tine ; oe, oesophagus ; reel, rectum ; siph, 

 siphon ; st, stomach ; w.v.r, water- vascular 

 ring. 



organisms. When a " handful," so to speak, of this nutritive 

 material has been collected, the long tube-foot is withdrawn 

 clown the burrow and passed over the deeply grooved part of the 

 ambulacrum to the buccal tube-feet, to which the food is given 

 up. These last then push it into the mouth. Only one pre- 

 hensile tube-foot is extended at a time. 



The stone-canal is very short and soon opens into the axial 

 sinus ; it is widely separated from the pore-canals which traverse 

 the madreporite. Communication between the two is effected by 

 the long axial sinus. There are only four genital organs. 



Heart-urchins vary amongst themselves chiefly in the foUow- 



^ Details were given to the author in conversation with Dr. Robertson in 1896. 



