NO. 19.] ECHINODERMS OF CONNECTICUT. 



Arbacia punctulata (Lamarck) 



Purple Sea-urchin 

 Plates XX, XXII, XXV. 



This is the only species of the regular sea-urchins common in 

 the greater part of Long Island Sound. It is widely distributed, 

 but occurs in abundance only in restricted localities. It prefers 

 shelly or rocky bottoms, and is often dredged from the oyster 

 beds. It sometimes occurs between tides in the tide pools in 

 rocky places, hiding in crevices or beneath seaweeds. 



The species occurs in shallow water along the coast from 

 Cape Cod to the Gulf of Mexico. There is great variation in 

 the abundance of individuals of this species in the same locality 

 in different years. At the Biological Laboratories at Woods 

 Hole, Mass., large numbers of these urchins are desired for 

 embryological and experimental studies each year, and, while 

 in some years the species can be found in unlimited numbers, 

 in other years it is extremely difficult to secure a supply. On 

 the oyster beds off New Haven Harbor, there seems to be a 

 similar variation in numbers. 



The purple sea-urchin is commonly from one to two inches 

 in diameter, excluding the spines. The spines are heavy and 

 solid, and measure from one-half to one inch in length. Each 

 is fluted longitudinally, and, while some of the upper ones are 

 acutely pointed, others are blunt. Towards the oral surface 

 they become shorter and flattened at the tip. They are usually 

 of a purplish color. The test is likewise reddish, purple, or 

 purplish brown, while the tube-feet are red or brown. 



EXPLANATION OF PLATE XXV. Purple Sea-urchin, Arbacia punctulata. 



(Natural size.) 



Fig. i. Aboral surface of test with spines removed, showing arrange- 

 ment of plates. The five double rows of dotted plates represent 

 the ambulacral areas, while the intervening rows of plates 

 comprise the interambulacral areas. The four plates at the 

 center form the periproct. Each of the interambulacral areas 

 ends in a plate with a small perforation, the genital pore. One 

 of these plates is somewhat larger than the others and serves 

 as the madreporic plate. 



Fig. 2. Side view of test. 



Fig. 3. Oral surface of test, showing the large opening covered in life 

 by the peristome. 



(From photographs loaned by the U. S. Fish Commission, with permission 



of Dr. H. L. Dark.) 



