No. 19.] ECHINODERMS OF CONNECTICUT. 85 



nearly circular in outline in the living animal, but often becomes 

 pentagonal when preserved in alcohol or dried. The aboral sur- 

 face of the disk is covered with numerous, small, naked, imbri- 

 cated scales, forming a rather distinct rosette at the center. The 

 radial shields are long and narrow, with their outer ends in con- 

 tact, and their inner ends separated by a narrow wedge of small 

 scales. The oral surface is covered with more minute scales 

 than those of the aboral surface. The oral shields are shield- 

 shaped, rather longer than wide. 



The five oral papillae are unequal in size, but all are obtusely 

 rounded. The inner one is the largest and stoutest, while the 

 next is the smallest and most acute; the next two are flattened, 

 while the outermost is small and rounded. 



The under arm plates are shield-shaped and longer than 

 broad. Near the base of the arms they are in contact, but farther 

 out they are separated by the side arm plates. Tentacle scales 

 two. Arm spines three, nearly equal, about as long as the joints. 



Color light gray. Diameter of disk about 4 mm. Length 

 of the extremely slender arms about 60 mm. 



CLASS III. ECHINOIDEA 



This class includes the animals popularly known as sea- 

 urchins, sand-dollars, cake-urchins, disk-urchins, sea-cakes, and 

 sand-cakes. In the sea-urchins the body is nearly hemispherical, 

 while the others are flattened and discoid. Spines thickly cover 

 the body on all sides, except sometimes at the poles. In one of 

 our species (Mellita pentapora) the disk is pierced by five oval 

 perforations. 



STRUCTURE 



If the purple sea-urchin (Arbacia punctulata) be taken as 

 an example of this group, the body is seen to be enclosed in a 



Explanation of Plate XX. Purple Sea-urchin, Arbacia punctulata. 



(Natural size.) 



The larger specimen (lower figure) shows the oral surface, with the 

 five sharp white teeth in the center; while the smaller specimen (upper 

 figure) shows the aboral surface, in the center of which is the periproct, 

 surrounded by a broad area free from spines. (After Clark.) 



