chap, xv.] RADIAL JOINTS OF COMATUL^E. 421 



One or more of the hypotheses are thus clearly at fault. A 

 natural, and I believe a true comment will occur to every one : 

 that probably the injured insects are not at any serious disad- 

 vantage, and that these mutilations perhaps make very little 

 difference to their chances. But can we admit that the loss of 

 a leg matters little, and still suppose that the definiteness and 

 accuracy of the exact proportions of the tarsal joints makes any 

 serious difference ? 



The hypothesis, therefore, that the smallness of the mean 

 variation in the proportional lengths of the tarsal joints of the 

 4-jointed tarsus has been gradually achieved by Selection is un- 

 tenable, whether that 4-jointed tarsus be a product of regenera- 

 tion or a congenital variation. But if the accuracy with which 

 the abnormal conforms to its type be not due to a gradual Selec- 

 tion, with what propriety can we refer the similar accuracy of 

 the normal to this directing cause ? 



Kadial joints in Arms of Comatul^e. 



The number of radial joints above the basals up to the division of the rays in 

 Crinoids is usually constant in the genera. In Antedon and Actinometra there are 

 normally three such joints, the third radial being the axillary, and none of these 

 bear pinnules. Both increase and decrease in the number of radials has been 

 observed, but variations from this number are rare, more so than variations in the 

 number of rays. Carpenter, P. H., Chall. Rep., xxvi. Pt. lx. p. 27. 



Antedon alternata: specimen having in one ray four radials, none bearing 

 pinnules or united by syzygy. ibid., PL xxxu.fig. 6. 



Encrinus gracilis (fossil) : in one ray four radials. Wagner, Jen. Ztschr., 

 1887, xx. p. 20, PI. ii. fig. 13. 



Antedon remota, A. incerta, Actinometra parvicirra (Fig. 126); one 

 specimen of each of these species had one ray with only two radials. Carpenter, 

 I.e., PL xxix. fig. 6; PL xviii. fig. 4; PL lxi. fig. 1. 



Fig. 126. Actinometra parvicirra, No. 628. Specimen having only two radials 

 in the ray marked x. (From P. H. Carpenter.) 



growth, as the other appendages are, but when formed again it is coiled up in a 

 tight conical spiral which cannot be extended at all, but is kept firmly in place by 

 the shortness of the skin on the inner curvature. (For figure see Howes, Jour. 

 Anat. Phys., xvi. p. 47.) During the process of regeneration the antenna is very 

 soft, and were it extended it would from its great length be much exposed to injury. 

 At the next moult after renewal the new antenna is drawn out as a straight filament 

 like the normal, and its skin then hardens with that of the rest of the body. This 

 strange manner of growth occurs only on regeneration. It is hard to believe both 



