^8 THE VOYAGE OF H.M.S. CHALLENGE!;. 



B. The Pinnules. 



The pinnules are repetitions of the arms on a small scale, and are especially adapted for 

 the protection of the genital glands (PL Vc. figs. 7, 8, 10, t ; PI. VII. fig. 7 ; PI. X. fig. 20). 



In no case is a pinnule developed earlier than the second joint above the first radials 

 of the calyx. This condition occurs in the two five-armed genera Thaumatocrinus 

 (PL LVI. figs. 1, 2) and Eudiocrinus. One species of the latter [Eudiocrinus varians) 

 has this second brachial free and capable of lateral movement, while in another 

 {Eudiocrinus indivisus) it is the epizygal of a syzygy. The corresponding radial joint 

 of Metacrinus is of the same character, and there are pinnules on each of the following 

 radials as far as the axillary (PL XXXVIII. ; PL XXXIX. fig. 1 ; PL XLIL; PL XLIII. 

 fig. 2 ; PL XLIV.; PL XLV. fig. 1 ; PL XLVL; PL XLVIII. fig. 1 ; PL XLIX. fig. 1 ; 

 Pis. L.-LII. fig. 1). But in the majority of Neocrinoids which have the third radial 

 an axillary, the preceding joint bears no pinnule, while it sometimes contributes to the 

 enlargement of the cup. 



Pinnules are always absent from every axillary joint, from the hypozygal of every 

 syzygy (PL XXXa. fig. lOh ; PL XXXII. figs. 4, 6, 13, 14; PL L. figs. 11, 12), and 

 also from the lower one of every pair of joints which are united by a ligamentous articu- 

 lation ; so that in the great majority of Comatulse, as in some sj^ecies of Pentacrinus, 

 the first joint after each axillary bears no pinnule. In the former group too the pinnules 

 on the third and the four or five following brachials which form the arm-bases, do not 

 appear till after those of the eighth and following joints, though the pinnule of the 

 second brachial is developed comparatively early ; while in Atelecrinus, Rhizocrinus, and 

 Bathycrinus more or fewer of the lowest brachials are permanently devoid of pinnules. 



The lowest pinnules of the Comatulse, and in a less degree those of the Pentacrinidas 

 also, usually ditfer somewhat from their successors ; and they may present a variety of 

 characters, which are of considerable value in the discrimination of species, owing to 

 the comparative constancy of their occurrence. They are frequently distinguished by 

 the presence of spurs or keels upon their basal joints, as in Actinometra Solaris ; or they 

 may be long, slender, flexible, and flageUiform, as in Antedon rosacea; or they may be 

 stifi", straight, and spine-like, as in Antedon 2>i"otecta; or they may have large prismatic 

 basal joints, as in Metacrinus (PL XXXVIIL; PL XXXIX. fig. 1 ; PL XLIII. figs. 2, 4 ; 

 PL XLIV. fig. 2; PL XLVL; PL XLIX. figs. 1, 2; PL L. figs. 1, 2 ; PL LIL fig. 1) ; 

 or the dorsal surfaces of their joints may have forward projecting keels, as in Pentacrinus 

 asterius (PL XIII. figs. 1, 14). 



Dr. Carpenter^ has observed that the first pinnules of the ten-armed Antedon rosacea, 

 which habitually arch over the disk and are much longer than their successors, are 



' On the Structure, Physiology, and Development of Antedon (Comatula) rosaceus, Proc. Roy. Soc, vol. xxiv., 1876, 

 p. 226. 



