no. 1845. ON THE CRINOID GENUS MARSUPITES— CLARK. 651 



gitudinal muscles. Only those echinoids which have flexible tests 

 possess longitudinal muscles; many of the species never develop 

 teeth, while some do not develop the lantern apparatus; several gen- 

 era of the Arbaciidse never develop spines upon the apical surface, 

 this permanently remaining as in the very young of Arbacia. 



We see, therefore, that it is a very common thing for a genus or other 

 group living under special conditions to suffer an arrest in the devel- 

 opment of some organ or set of organs, and in very many cases, best 

 illustrated among the echinoderms, perhaps, by the well-known 

 instances among the Arbaciidas (Arbaciella, Podocidaris, Dialithoci- 

 daris, etc.), this arrest in development is chiefly evident in the very 

 primitive condition of the general body form, as shown by a compari- 

 son of the adults of these genera with the very young of Arbacia. 



In the very young of Antedon, as has been demonstrated by Bury 

 and by Seeliger, there are: 



(1) An apical system, consisting of a central plate, or dorsocentral, 

 and a few columnars; 



(2) A circlet of underbasals surrounding this apical system; 



(3) A circlet of basals outside of and alternating with the under- 

 basals; 



(4) A circlet of radials beyond and alternating with the basals. 



(5) A circlet of orals, each oral lying directly bej^ond a basal. 

 The differences between the calyx of the young Antedon and that 



of Marsupites are : 



(1) In Marswpites the apical s} r stem is composed of a single plate 

 instead of one large plate and a series of smaller ones; 



(2) In Marswpites the underbasals are very large, as large as the 

 basals, instead of being very small, as in Antedon. 



This latter difference is easily shown to be of no especial significance 

 by a study of the calyx development among the recent crinoids, and 

 especially among the comatulids. Primarily the calyx is composed 

 of four circlets of plates, all the plates being approximately equal in 

 size; these circlets are (1) the underbasals, (2) the basals, (3) the 

 radials, and (4) the orals. Phylogenetic development is in the 

 direction of a reduction in the size of the calyx cavity and of a pro- 

 gressive extrusion of the visceral mass. The underbasals are first 

 affected; instead of forming, as in the primitive condition, an impor- 

 tant integral part of the body wall, they move inward, reducing their 

 circlet in size, so that their inner borders, instead of abutting on the 

 sides of the topmost columnar, slip inward over it; at the same time 

 they gradually become more and more recumbent, finally attaining a 

 horizontal position and, instead of functioning as a part of the lateral 

 calyx wall, become merely a sort of flooring upon which the contents 

 of the calyx rest. The basals are next affected; they become reduced 

 in size, and their lower edges slip inward over the inner side of the 



