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THE AMERICAN NATURALIST [Vol. XLIII 



never bear spines; (12) the ambulacrals are not con- 

 tinuous with, nor in the same plane as, the so-called 

 radials or terminals. 



This would be sufficient in itself to convince any one 

 that the crinoids and echinoids formed one well circum- 

 scribed group, while the asteroids and the ophiuroids 

 formed another similar group, entirely distinct ; but the 

 "soft parts" furnish abundant additional evidence lead- 

 ing to the same conclusion. 



In the urchins and the crinoids the anus is always well 

 developed and functional, while in the asteroids and 

 ophiuroids it is absent, or, if present, does not serve as 

 an exit for refuse matter ; the crinoids and urchins have a 

 large and definite peristome which is circular in shape 

 and more or less filled with dermal plates ; in the aster- 

 oids and ophiuroids the peristome is very much reduced, 

 stellate and without dermal plates; in the crinoids the 

 ambulacral plates are united by ligaments externally and 

 by two parallel rows of muscle bundles internally; in the 

 echinothurids, which alone among the echinoids have a 

 flexible test, the plates are united by more or less liga- 

 mentous connective tissue, and within the test there are 

 five pairs of muscle sheets, inserted along the two outer 

 edges of the ambulacral series ; the asteroids and ophiu- 

 roids have two pairs of muscles, a dorsal and a ventral 

 instead of one pair and a dorsal (external) ligament. In 

 the crinoids and echinoids the intestinal canal is narrow, 

 tubular, without marked sac-like expansions, and always 

 lies in coils of which there may be as many as four; in 

 the asteroids and ophiuroids the digestive system runs 

 direct from the mouth to the anus (when present) with- 

 out convolutions, but has sac-like widenings, and may 

 have branched radial outgrowths. In the crinoids and 

 the echinoids the gonads are connected with the axial 

 organ in the young, but not in the adult; in the asteroids 

 and ophiuroids they are connected with the axial organ 

 throughout life. In the echinoids and crinoids the 

 pseudo-haemal system is closed on all sides; in the as- 

 teroids and ophiuroids it connects by means of numerous 



