108 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [BuU. 



The aboral pole is free from spines for a considerable dis- 

 tance; not only are the plates of the periproct and the genital 

 plates naked, but spines are likewise absent from the middle of 

 the interambulacral areas for a distance nearly half-way to the 

 equator of the test. Only the large primary spines are present. 

 They are arranged in a double series in each ambulacral area, 

 and in four to eight or more series in each interambulacral area. 

 There are five pairs of prominent buccal plates. 



In Connecticut waters the sexual products are matured in 

 early summer. The eggs contain particles of the purplish pig- 

 ment which permeates nearly all the tissues of the body. They 

 are admirable objects for the study of the processes of fertili- 

 zation, cleavage, and early embryonic development. 



For such a study the suggestions given for the examination 

 of the starfish egg will apply equally well. The free-swimming 

 embryo, known as the pluteus, may be kept alive for a number 

 of days, but nearly all attempts to rear artificially to the adult 

 form have failed. 



Strongylocentrotus drobachiensis (O. F. Miiller) 

 Green Sea-urchin 

 Plates XXI, XXVI. 

 This is a typical drcumpolar species, occurring in great 

 abundance north of Cape Cod, and is found but locally in Con- 

 necticut waters. It extends southward in the deep water off our 

 coast as far as New Jersey, but south of Cape Cod it is of 

 much smaller size than northward. 



In Long Island Sound small specimens have been dredged off 

 New Haven and Stratford; but these individuals or their an- 

 cestors have presumably been carried in their early stages by the 



ExPLANATidN OF PLATE XXVI. Green Sea-urdhin, Strongylocentrotus 

 drobachiensis. (Natural size.) ' 



The upper figure shows the aboral surface, thickly covered with the 

 primary and secondary spines. 



In the center of the aboral surface (lower figure) is seen the peristome 

 with the five sharp white teeth which close the mouth. Between the 

 spines are shown the five double rows of slender tube-feet with enlarged 

 terminal sucking disks. 



(Photographs loaned by the U. S. Fish Commission, with permission of 

 Dr. H. L. Dark.) 



