100 



SYSTEMATIC ZOOLOGY 



diameter, but along the Pacific coast they have been found with 

 a shell some fifteen centimeters broad and spines half as long, 

 so that the animal measured some thirty centimeters. Dr. 

 Alexander Agassiz has described a species which he dredged, 

 in which the shell measured 312 millimeters. There is much 

 variety of color in this group, some species being green, others 

 purple, and some yellowish and pinkish. Those which live in 



deeper water show much diversity 

 of form ; the spines are sometimes 

 very long and greatly thickened, 

 sometimes broad and platelike, and 

 often highly ornamented by fiutings 

 and ridges. Some species have 

 curious hollows or pouches, in the 

 shell, partly covered by spines, 

 and these serve as brood pouches, 

 where the young undergo a certain 

 part of their development. Some 

 oi these species which have such 

 striking bilateral symmetry exhibit 

 a considerable variation in the 

 positions of the mouth and anus. 

 The mouth no longer occupies the 

 center of the oral surface, but has 

 moved towards what we may call 

 the anterior end, and the anus 

 comes to lie on the oral surface of 

 the animal instead of on the margin 

 or on the aboral side, as in the 

 more distinctly radially symmetrical species. The number of 

 living species of sea urchins is very small compared with the 

 number of fossil species. 



Fig. 92. Arbacia punctata, a sea 

 urchin; pluteus larva, ventral aspect, 

 showing larval skeleton; somewhat 

 diagrammatic and highly magnified. 

 (Drawn from life by the author.) 



CLASS V. HOLOTHUROIDEA 



The Holothuroidea (Gr. 6'\(K, whole, and OvpoeiBtjs, like a door) 

 consist of the sea cucumbers. The common name for these 

 animals is verv applicable in many cases, for they are often more 

 or less cylindrical, and with little evidence of that radial arrange- 

 ment of parts which is so characteristic of the other class - 



