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 1 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 

 finger-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 Hemiaster Fig. 247. Dissection of Echinocardium cor- 



philippi, showing 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 ; red, 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 material has been collected, the long tube-foot is withdrawn 

 down 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 follow- 



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





