46 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



arms of the larger Pentacrinoids (plate xxi, figure 6), viz, the primary, 

 azygous tentacle, situated in the cleft of the arm. It remains here and must 

 ultimately become absorbed. This fact was already observed by Wyville 

 Thomson in Antedon bifida.* 1 Plate xxi, figure 7, which is drawn from a 

 preparation of a Pentacrinoid of Antedon bifida, elucidates this point well. 

 It is seen here how the radial canals of the arm-branches originate as side- 

 branches from the primary radial canal a little below the free azygous 

 tentacle. The figure also shows that the distal tentacle is the first to develop 

 in each triplet of tentacles. 



6. DEVELOPMENT OF THE SKELETON; THE PENTACRINOID. 



The first stage of the skeletal development is represented in plate xxn, 

 figures 4 and 5. The oral and basal plates have the shape of branched spiculcs 

 arranged in two circles, the plates of the basal circle lying almost exactly 

 below the corresponding ones of the oral circle. The terminal stem-plate 

 has been formed and some few stalk-joints are indicated by very small 

 spicules, 3 in figure 5, 7 in figure 4. Infrabasalia are not seen, nor is there 

 any trace of them in the later stages, so it is evident that they are entirely 

 lacking in this species. In figures 6 to 8, plate xxn, which represent the 

 fully formed larva, the oral and basal plates have become large, fenes- 

 trated plates, arranged in two regular half-circles, leaving a broad, open 

 space on the ventral side (plate xxn, figure 6) ; they are distinctly separated 

 from one another in the middle, where the constriction of the larva takes 

 place, the oral plates thus occupying the posterior and the basal plates the 

 anterior end of the larva (plate xxn, figure 7). The terminal stem-plate 

 is large and fenestrated, and the stalk-joints have augmented considerably 

 in size and number; their exact number is hard to ascertain, because they 

 lie so close together. The plates of the fully formed larva are, upon the 

 whole, strongly developed, and almost convey the impression that the larva 

 is a mail-clad little organism, the naked median constriction apparently 

 affording the line of motion between the mailed anterior and posterior part. 

 As seen in the three figures quoted, the larva in this stage is still inclosed by 

 the egg-membrane, which does not burst till the moment when the larva 

 leaves the marsupium. 



The newly attached Pentacrinoid (plate xxn, figure 9; plate xxni, 

 figure 1) is a curious little, short-stalked, thick-headed organism, the oral 

 and basal plates inclosing it now very completely. The stalk-joints are 

 still very short and so closely packed that it is impossible to count them. 

 The upper third part of the stalk is still inclosed by the basals. As seen in 

 plate xxni, figure 4, it happens that the young Pentacrinoids attach them- 

 selves to the stalk of their slightly elder sisters and brothers. The first 



31 Wyville Thomson. On the Embryogeny of Antedon rosaceus Linck. Phil. Trans., vol. 155, 1865, p. 259. 



