60 THE VOYAGE OF H.M.S. CHALLENGER. 



enlarged to receive the fertile portions of the genital gknds, no part of these appearing 

 in the arms (PI. Vc. figs. 7, 8, 10, t; PI. VI. fig. 1); while they have no appendages of 

 their own as the armlets have in Extracrinus. 



The peculiar pinnule arrangement of Hyocrinus helps us to understand why there 

 are no pinnules upon the axillaries of multiradiate Crinoids. These may be considered 

 as ordinary pinnule-bearing joints, so modified that the pinnule and the continuation of 

 the arm which bears it are equal in size or nearly so. As mentioned above, this is in 

 fact the mode of formation of the pinnules at the growing points of the arms, as is well 

 shown in a very young individual of Pentacrinus decorus (PI. XXXV. fig. 1). The 

 joint which bears the last formed pinnule is an axillary with two nearly equal distal 

 faces ; and the pinnule can only be distinguished from the continuation of the arm by 

 the greater length of its component joints. Furthermore, in the short posterior arms of 

 Actinometra, the only ones in which the normal mode of termination has been observed,' 

 the last joint is an axillary which bears two pinnules of the ordinary character. 



In Rhizocrinus (PL IX. figs. 4, 5) as in Hyocrinus (PI. VI. figs. 1, 2) the pinnule- 

 bearing joints have very much the appearance of axiUaries with unequal distal faces; and 

 a similar inequality is shown by the axillaries of Extracrinus, each of which bears an 

 " armlet " on one face and the continuation of the main arm -trunk on the other. 



Numerous instances of reparation after injury also indicate the close similarity of 

 arms and pinnules. A very common one, sometimes to be met with in Antedon rosacea, 

 is as follows : — The epizygal of the third brachial is broken away, carrying with it all the 

 outer part of the arm, as well as the pinnule which it bears. But it is replaced by an 

 axillary with two distal faces, from each of which an arm eventually grows out, one or 

 other of them perhaps dividing again, as in the specimen of Pentacrinus decorus shown 

 on PL XXXVI. On the other hand, in an abnormal individual of Metacnnus angidatus, 

 the eighth distichal is not an axillary, as is usually the case. But it is somewhat swollen 

 and has a slightly larger pinnule than the preceding joint, so that it resembles an axillary 

 with unequal faces. In the specimen of Actinometm strota which is represented on 

 PL LV. fig. 2, one of the second brachials of the right posterior ray bears two fully 

 developed pinnules instead of an arm and its own proper pinnule, so that it looks like an 

 axillary. There is no disk-ambulacrum corresponding to this undeveloped arm. 



Considering therefore the fundamental identity of arms and pinnules, one w^ould 

 scarcely expect that an axillary joint which gives rise to two arms (often unequal in size) 

 should bear a pinnule as well (see pp. 347, 358). 



The pinnule arrangement of Hyocrinus is totally unlike that of any other Neocrinoid, 

 although, according to Sir Wj^irille Thomson," we have something very close to it in some 

 species of the Palseozoic genera Poteriocrinus and Cyathocrinus. These names were 

 probably employed by Sir Wy ville in the ■wide sense, and not with the restricted meaning 



' Actinometra, loc. cit., p. 40, pi. ii. fig. 6. ^ Journ. Linn. Soc. Lend. (ZooL), vol. xiii., 1876, p. 52. 



