MONOGRAPH OF 1HE EXISTING CBINOIDS. 341 



species of Calometridte, five small orals are often found which are apparently the 

 same as the orals of the young. 



These seem to be in reality, however, secondaiy perisomic orals, oral-like 

 perisomic plates developed in the apex of each interradial area exactly as the 

 covering plates are developed in the marginal lappets bordering the ambulacral 

 grooves, and to have no connection whatever with the true orals of the young. 



The relation between the true orals and the secondary perisomic orals in these 

 forms appears to be the same as that between the true interra dials of the young of 

 the comasterids or of the species of Antedon and the perisomic interradials of the 

 adults. 



General proportions of calyx and its contents. 



The calcareous investment of the echinodenns reduced to its simplest and 

 most primitive form, as explained in the section dealing with the skeleton in gen- 

 eral, was a diffuse spicular development in the body wall; fusion of these spicules, 

 governed by mechanical localization, gave rise to a ring of more or less definite 

 plates, five larger, midsomatic (interradial) in position, and five smaller, interso- 

 matic (radial) in position, about the anterior end of the digestive tube. There is a 

 possibility, amounting almost to a probability, that the plates of this circumoral 

 ring are not morphologically related to the spicular skeleton of the rest of the 

 animal except in a very general way, but are plates inherited as such from the 

 prototype of the group. 



This ring, whatever its ultimate' origin, moved away from its primitive posi- 

 tion about the anterior part of the digestive tube, passing around to the posterior 

 part of the body, where it came to form a circlet of plates about the dorsal apex, a 

 second newly formed ring appearing in its original position; the path taken by each 

 plate of the original ring over the body wall was marked by a series of repetitions 

 of the plate which were continually formed at its proximal border as it moved 

 along. 



The second ring underwent the same course of development as the first; it, too, 

 moved outward; and in the crinoids we find it, in the form of radials from which 

 long and complex post-radial series arise, superposed, through the gradual disap- 

 pearance of the trail of plates left by the first in its passage, directly upon the 

 original plates of the first, while a third ring has taken its place about the mouth. 



As we understand it, the original calcareous covering of the body after the 

 true crinoid type was attained took the form of a more or less globular capsule 

 composed of: (1) a central plate or cemrale, usually lengthened out into a long 

 column by a process of continual reduplication, more rarely represented by scat- 

 tered perisomic plates and spicules in the apical area; (2) a circlet of five inter- 

 somatic plates, the infrabasals, immediately surrounding the centrale or resting 

 upon the summit of the column, each of which serves as the base of a complex 

 series of ambulacral ossicles; (3) a circlet of five larger midsomatic plates just 

 beyond the infrabasals and alternating in position with them, the basals, each of 

 which serves as the base of a series of interambulacrals; each of these basals is 

 separated from its neighbors on either side by the first ambulacral plate following 



