170 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



concentration and rearrangement of the plates and their solidification is accom- 

 panied by an enormous reduction in their total mass, so that the column has a 

 much lessened weight to support. 



The crinoid is most nearly related to the echinoid, but possesses certain features 

 both of the asteroid and of the ophiuroid, so that it is to a considerable degree 

 intermediate between them. The characters which link the crinoids to the echi- 

 noids on the one hand, and to the asteroids and the ophiuroids on the other, are all 

 most evident in the older forms; and in these we find the characters connecting 

 the crinoids and the echinoids more pronounced and more significant than those 

 connecting the crinoids with the asteroids and the ophiuroids. In the later types 

 and in all the recent forms the connection with the echinoids has, owing to the 

 increasing proportionate size of the five radial processes of the body and the corre- 

 lated proportionate great reduction in the size of the body proper, become largely 

 obliterated, while the traces of the connection with the asteroids and with the ophiu- 

 roids have not been subjected to anything like the same degree of suppression. 



AH the plates of the crinoids and echinoids appear to have been derived from 

 the circumcesophageal plates of the holothurians except the auricles and associated 

 plates in the echinoids, and the brachials beyond the third in the free arm corre- 

 sponding to them, and the orals, in the crinoids. 



The fundamental plate series in all the echinoderms thus appear to reduce 

 themselves to rings of plates around the mouth, or at least about the anterior 

 portion of the digestive tube one in the holothurians, two in the echinoids, and 

 three in the crinoids. 



It remains to be seen whether any homology may be found for these successive 

 rings of plates among bilaterally symmetrical invertebrates. 



These plates consist of five larger, in the midsomatic areas, and usually also 

 five smaller, in the intersomatic regions, though the latter may be absent as hi the 

 oral ring of the crinoids and in the coronal ring of the blastoids and of the so-called 

 monocyclic crinoids. 



In the echinoids there is commonly developed in connection with the second 

 ring system, the auricles, an extremely complicated structure known as the "Aris- 

 totle's lantern," consisting of five dental pyramids, each surmounted by a powerful 

 tooth. 



In the insects and crustaceans there is usually developed on at least one of the 

 anterior somites a pair of powerful mandibles, which may be either wholly chitmous 

 or partially calcareous. These mandibles are usually associated with the anterior 

 end of the digestive tube more intimately than any other of the mouth parts. 



All the somites of the echinoderms are exactly alike; any structure occurring 

 in one may, and usually does, occur similarly developed in all the others. In the 

 rearrangement by which the echmoderms were evolved from their bilateral ances- 

 try the mandibles and their braces, the most significant of all the mouth parts, 

 were retained potentially in their original relationship. There being five somatic 

 divisions about the mouth, the mandibular structures when present are always 

 repeated five tunes, each of the repetitions being similar to each of the others. 



