ARBACIA PUNCTULATA Lamarck 



Material. Arbacia punctulata is found in shallow water 

 and in tide-pools along the Atlantic Coast. It is advisable to 

 make an incision either in the peristomial membrane or in the 

 equatorial line in those specimens which are to be preserved in 

 formalin or alcohol for dissection. 



Descriptive Part 



Arbacia punctulata, or purple sea-urchin, is a common repre- 

 sentative of the Class Echinoidea. With exception of a small 

 area on the back and a somewhat larger one on the ventral side, 

 the entire body is covered with spines. In the centre of the 

 dorsal or aboral surface lies the anus protected by four calcareous 

 movable plates of the periproct. In the centre of the ventral 

 or oral surface lies the mouth armed with five sharp white teeth. 

 The mouth can be tightly closed by a circular lip which is a 

 muscular differentiation of the soft peristomial membrane. The 

 edge of this membrane is attached to the test or rigid calcareous 

 skeleton. Around the lip are five pairs of modified ambulacral 

 tubes the oral papilla or suckers. Scattered over the peristome 

 are long-stemmed pedicellaria. On the edge of the peristome, 

 attached to the peristomial membrane are five pairs of branched 

 organs of respiration the branchice. Five clusters of ambulacral 

 feet surround the peristome. 



When the spines are removed the test appears in the shape of 

 a hemisphere with a rounded edge. It is composed of twenty 

 rows of interlocking calcareous plates extending from the peri- 

 proct to the peristome. Five pairs of rows consist of perforated 

 ambulacral plates and represent the radii, while the five pairs 



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