MONOGRAPH OF THE EXISTING CRINOIDS. 347 



supporting and protective functions originally exercised by the infrabasals; basals 

 and radials have, in the comatulids and in tlie pentacrmites, been assumed by the 

 postradial ossicles to and including the second brachial of the free undividetl arm. 



As the visceral mass has constantly ijicreased in proportionate size, while the 

 basals have dwindled and become metamorphosed into the rosette, and the radials 

 have ceased their development and become small recumbent plates, it now projects 

 far outward on everj^ side and has come to bo supported upoti the IBr series and the 

 first two bracliials, which have assumed the lateral supporting and protective 

 functions originally and primarily characteristic of the basals, radials, and other 

 calyx plates (figs. 83, p. 136, 85, p. 139, 92, p. 151, 111, p. 177, 113, p. 181, 119, 

 p. 185, and 121, p. 189). 



Thus the calyx of the comatulids is peculiar in being primarily made up of 

 three circlets of horizontal plates alternating in position and superposed one upon 

 the other, the uppermost circlet forming the floor upon which the visceral mass 

 rests, the calj^x plates having entirely lost their original function of inclosing 

 and protectmg the visceral mass, one circlet having disappeared or become quite 

 obsolete (the infrabasals), the next having been so metamorphosed as to per- 

 form the duties merely of an undivided horizontal septum within the original 

 caljTC (the basals), and the outer (the radials) having been so reduced as to serve 

 practically no other purpose than as a base for the attachment of the arms (figs. 431, 

 432, p. 349). 



In regard to the changing relations between the calyx plates and the visceral 

 mass in the developing young of Antedon bifida, W. B. Carpenter sa3's: "For 

 some little time after the appearance of the arms the relations of the skeleton of 

 the calyx to the visceral mass it includes undergoes but Utile change, the chief 

 difference consisting in the more compact condition it now comes to present in 

 consequence of the advanced development of its component pieces. The five 

 basals now possess a regularly trapezoidal form, the lower part of each being an 

 acute angled triangle with its apex pointing downward and its upper part an 

 obtuse angled triangle with its apex directed upward. The radials, with the 

 anal intercalated between two of them, now form a uearty complete circle resting 

 on the basals and separating them entirely from the orals. Their shape is some- 

 what quadrangtdar, two of their angles pointing vertically upward and dowiiward, 

 the other two laterally toward each other. Their lower angles are received between 

 the upper angles of the basals. A very important change takes place in the rela- 

 tions of the several parts of the cah-x and its contents which gives to the body of 

 the more advanced pentacrinoid a much closer resemblance to that of tho adult 

 Antedon. Instead of being completely included within a calcareous casing, wliich not 

 only supports it below but can close over it above, tho visceral mass which occupies 

 the cavity of the calyx, is henceforth to bo merely supported by its skeleton, its 

 upper surface losmg all protection except such as is afforded bj' the infolding of 

 the arms, and being extended ijito a disk of which the mouth only occupies the 

 center. Tliis change is essentially connccteil with the increased development of 

 the intestinal tube which now forms a nearly complete circle around the stomach 



