MORPHOLOGY. 



L— THE SKELETON GENERALLY, WITH THE MODES OF UNION OF 



ITS COMPONENT JOINTS. 



The organisation of a Crinoicl is broadly divisible into two well-marked portions, to 

 which the general names ambulacral and antiambulacral may be given. They correspond, 

 on the whole, to the left and right larval antimers respectively, though probably not 

 exactly so. The first is the visceral mass or " disk," in which is situated the whole of 

 the digestive tube, with both its terminal openings. It likewise contains the central 

 ends of the radial water-vessels and blood-vessels, wliich converge towards their 

 respective circum-oral rings, and also the corresponding portions of the ambulacral 

 nervous system (PI. LXIL). 



Both the disk and its extensions in the perisome clothing the ventral surface of the 

 arms and pinnules are usually more or less covered by calcareous plates, the arrangement 

 of which will be described subsequently (PI. VI. fig. 4; PL XVII. figs. 6-10; PI. XXYI. 

 figs. 1, 2; PL XXXIIL figs. 6, 7; PL XXXIX. fig. 2; PL XLL figs. 4, 12-14; PL XLHL 

 fig. 3 ; PL XLVII. figs. 10-13 ; PL L. fig. 2 ; Pis. LIV., LY.). They represent a portion 

 of that element of the Crinoid skeleton to which the name " perisomatic " was given by 

 Sir Wyville Thomson^; for they are all originally developed from simple cribriform films 

 of limestone, such as appear in all young Echinoderms, and thicken by continual repetition 

 of the same formation. 



The antiambulacral portion of a Crinoid consists of the stem and its appendages, 

 the calyx, and the skeleton of the rays, arms, and pinnules. This constitutes the radial 

 skeleton, as the term is understood by Dr. Carpenter,^ A^iz., that which is perforated b)^ a 

 central canal lodging an extension of the fibrillar envelope around the chambered organ 

 (PL Yllb. fig. 2; PL XXIY. figs. 2-6, ca; figs. 7-9, ar; PL LYIIL figs. 1-3, ar ; 

 PL LXIL). 



Sir Wyville Thomson was unaware that the primary interradial cords proceeding from 

 the chambered organ perforate the basal plates (PL Yllb. fig. 2 ; PL XXIY. fig. 7 ; 



1 On the Embryogeny of Antedon rosaceus, Linck, Phil. Trans., 1865, pp. 540, 541. 



2 Researches on the Structure, Physiology, and DevL-lopnieut of Antedon rosaceus, part i., Phil. Trans., 1866, 

 p. V42. 



(ZOOL. CH.VLL, EXP. — PART XXSII. — 188-1.) K 1 



