1-2 EDWARD VII. SESSIONAL PAPER No. 22a 



A. 1902 



Y 



FOOD OF THE SEA-URCHIN (Strongylocentrotus drobachiensis.) 



BY Dr. F. H. SCOTT, Ph.D., PHYSIOLOGICAL LABORATORY, 

 UNIVERSITY OF TORONTO. 



The sea-urchin is one of the commonest animals on our Atlantic coast where great 

 numbers are found in all suitable places. They prefer a gravelly or rocky bottom and 

 are rarely found on mud or coarse sand. Just below the low tide mark on a gravelly 

 beach, or better on a beach of medium-sized stones separated by patches of sand, the 

 sea-urchins are exceedingly numerous. Another favourite resort of the sea-urchin is on 

 the sides of bare rocks and reefs, where there are often thousands aggregated together. 

 Many, especially small urchins, are found under stones on the bottoms of tide pools. 

 Urchins frequently attach shells and other debris to themselves and in localities where 

 such materials are abundant are often invisible owing to such a covering. In the deeper 

 waters of Passamaquoddy Bay they are also abundant on suitable bottoms, for the 

 dredge is often filled with them from depths of 12 to 15 fathoms. 



The sea-urchin is more or less hemispherical in shape and is covered with movable 

 spines. The spines are green in colour, nearly an inch long and are articulated to the 

 shell or test by a ball and socket joint. The test, which after the removal of the spines 

 has well been likened by Ganong ^ to an old-fashioned smooth doorknob, is made of 

 twenty rows of hexagonal plates closely cemented together. Five double rows of these 

 plates are perforated and alternate with similar imperforate rows. On the external sur- 

 face of all the plates are little conical elevations which fit into depressions on the base 

 of the spines forming the movable articulations. Scattered among the spines are other 

 shorter appendages which end in minute pinchers (pedicellarise). These probably assist 

 the animals in grasping small objects. 



Within the test among the other organs is the water vascular system. This system 

 is peculiar to the Echinodermata and has the function of forcing water into the tube 

 feet, or of withdrawing it from them. The tube feet, which project through the open- 

 ings in the perforated plates of the test, are hollow cylinders capable of great extension. 

 Each foot ends in a sucker and thus the animal by attaching its feet is enabled to adhere 

 to different objects. When the water is forced in, the feet may extend away beyond the 

 tips of the spines ; but when the water is withdrawn the feet are much the shorter. 



The tube feet are the principal means of locomotion, although the animal can move 

 on its spines alone. By extending its feet on one side, attaching the suckers and then 

 pulling, the animal can move in any definite direction along flat surfaces or ascend per- 

 pendicular ones. By this method, two sea-urchins, in a tide pool with a smooth rocky 

 bottom, were observed to move six and seven inches respectively in two minutes. This 

 is at the rate of about sixteen yards per hour and indicates that the urchins might move 

 considerable distances during a tide period. Whether the urchins do move at every 

 tide is another question. A few observations lead me to think that they do not move 

 very much, but no experiments were made to decide this point. 



The usual position of the animal is with the flat side of the hemisphere towards 

 the ground. The central part of this side is membranous and devoid of spines. The 

 mouth is situated in the centre of this membrane and has the tips of the five teeth pro- 

 jecting from it. Only the tips of the teeth project outside, the remainder along with a 

 complicated apparatus for moving them being beneath the membrane. The oesophagus 

 a longitudinally ribbed tube leads to the intestine, there being no stomach such as is 

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