94 



CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



The respiratory tube-feet which mark the outlines of the 

 petals, as stated above, are much larger than the others, and 

 are bilobed at the base, with two fine tubes which pass through 

 the plates of the test in the double pores of the petal. Both kinds 

 of tube- feet bear suckers. 



Correlated with this highly modified arrangement of the tube- 

 feet is a corresponding specialization of the water-vascular sys- 

 tem, for each foot is connected with a branch of these vessels. 



MacBride* offers the following explanation of this complex 

 system : " The condition of the water-vascular system is to be 

 explained entirely by the peculiar environment of the animal. 

 The demand for specialized respiratory organs is brought about 

 by the habit of living half buried in the sand. Under these 

 circumstances the strain of supplying the needful oxygen is 

 thrown on the dorsal tube-feet, and they become modified in 

 order to fit them for this function. The locomotor tube-feet are 

 very small and feeble compared with those of Echinus esculentus; 

 but this is comprehensible when it is recollected how little resist- 

 ance the yielding sand would offer to the pull of a powerful 

 tube-foot like that of the regular urchins, for in order to move 

 the creature through the sand a multitude of feeble pulls dis- 

 tributed all over its surface is necessary, and the locomotor tube- 

 feet are exactly fitted, both as to size and number, for this 

 object." The power of movement is not wholly dependent 

 upon the action of the tube-feet, however, for the vast number 

 of minute spines which cover the body are freely movable and 

 undoubtedly aid largely in the process of locomotion. 



The structural weakness due to the flattened shape of the 

 body in the disk-like urchins is compensated by strong vertical 

 plates of calcareous material which unite the oral and aboral 

 parts of the test so firmly, especially towards the edges, that it is 

 difficult to break open the body except in its central portion. 



Internal Anatomy. — The alimentary canal shows consid- 

 erable modification in the disk-urchins, owing to the nature of 

 the food and the position of the anal opening. Dissections show- 

 ing the digestive tracts of the sand-dollar and the key-hole 

 , urchin are illustrated on Plate XXIV. 



* Cambridge Natural History, Echinodennata, page 547. 



