NO. 19.] ECHINODERMS OF CONNECTICUT. 91 



of the upper part being specially adapted for respiration, while 

 in Strongylocentrotus all the tube-feet have suckers. 



Nervous System. — As in the starfish, there is a main nerve 

 ring surrounding the mouth, with five radial nerves which ac- 

 company the radial vessels to the base of the terminal tentacle. 

 Each tube-foot and all the spines and pedicellarise are supplied 

 with branches of these radial nerves. 



The tube-feet are not only used for locomotion and respira- 

 tion, but are highly sensitive to various kinds of stimuli. Spe- 

 cialized tube-feet on the peristome enable the animal to taste. 

 The pedicellarige are in many species modified to perform a 

 variety of functions, some having specialized sense organs and 

 poison glands. Minute organs scattered over the body in many 

 species, and known as sphaeridia, are thought by some to act as 

 balancing organs. 



Reproductive System. — Attached by strong mesenteries to 

 the upper wall of the test are the five sexual glands. The genital 

 duct leading from each of these glands opens by a minute pore 

 in the corresponding genital plate adjacent to the periproct. 

 When fully developed the sexual glands become enormous masses 

 of tiny follicles (Plate XXII). 



The sexes are separate, but are not distinguishable externally. 

 The females produce many thousand eggs, which are discharged 

 into the water when mature. 



Comparison of Arbacia with other Genera. — A comparison 

 of our three other representatives of this class with the descrip- 

 tion given above for Arbacia offers many points of interest. 



The green sea-urchin agrees with the purple sea-urchin in 

 having a nearly hemispherical test, with the mouth placed sym- 

 metrically at one pole and the anal opening at the other. Both 

 species therefore belong to the order Regularia, or radially sym- 

 metrical urchins. Both the sand-dollar and the key-hole urchin, 

 on the other hand, have flat, discoid bodies, nearly circular in out- 

 line but with one or more slight indentations; and the key-hole 

 urchin has five narrow oval perforations (Plate XXVIII). 



In the sand-dollar and the key-hole urchin, the mouth occu- 

 pies the usual position in the center of the oral surface; but in 

 the sand-dollar the anus has been shifted to the edge of the 



