MORPHOLOGY. 
L—THE SKELETON GENERALLY, WITH THE MODES OF UNION OF 
ITS COMPONENT JOINTS. 
The organisation of a Crinoid 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, which converge towards their 
respective circum-oral rings, and also the corresponding portions of the ambulacral 
nervous system (Pl. 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. Wine. 4s ,P1 XVII. figs. 6-10; Pl. SOAW IL 
figs. 1,2; Pl. XXXIIL figs. 6,7; Pl. XXXIX. fic. 2; Pl. XLI. figs. 4, 12-14; Pl XLII. 
fir. 3; Pl. XLVIL. figs, 10-13; Pl. L. fig. 2; Pls. LIV., LV.). 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,’ viz., that which is perforated by a 
central canal lodging an extension of the fibrillar envelope around the chambered organ 
(Pl. VIIb. fig. 2; Pl. XXIV. figs. 2-6, ca; figs. 7-9, ar; Pl. LVIIL 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. Wilb. ‘fig. 2; PL XXIV S nee; 
1 On the Embryogeny of Antedon rosaceus, Linck, Plul. Trans., 1865, pp. 540, 541. 
2 Researches on the Structure, Physiology, and Development of Antedon rosaceus, part i., Phil. Trans., 1866, 
p. 742. 
(z00L. CHALL, EXP.—PART xxxi1,—1884.) Til 
