CHAP, xvii.] RADIAL SERIES I HOLOTHURIOIDEA. 433 



a particular ray is reduced in size (Nos. 680 and 681, &c.). There 

 are also two cases of imperfect division of a ray in an Echinid 

 (Nos. 688, &c.), while in Asteroids &c. this condition is common. It 

 is of importance to observe that just as in Linear Series abnormal 

 divisions of members of the series are commonly transverse to the 

 lines of Repetition, so in radial forms the divisions of rays are 

 commonly radial. 



The evidence is complicated by the fact that in many 

 Echinoderms extensive regeneration can occur, and in some 

 genera reproduction by division of the disc and subsequent 

 regeneration is almost certainly a normal occurrence 1 . Never- 

 theless it cannot be doubted that the variation seen in Echini, 

 in Asterina, in the discs and stems of Crinoids, &c., are truly 

 congenital. Similarly, though in Asterias &c. reduction in the 

 number of arms might otherwise be thought to be due to mutila- 

 tion, it cannot be so in Echini &c. 



HOLOTHURIOIDEA. 



*642. Cucumaria planci : among 150 half-grown specimens found 

 at Naples five were 6-rayed. LUDWIG, H., Zool. Anz., 1886, ix., 

 p. 472 [These specimens are described in detail.] To determine 

 which is "the intercalated ray" the following ingenious reasoning 

 is offered, and as a good practical illustration of the conception 

 of the individuality of segments as applied to an Echinoderm 

 we may well consider it. 



1 It is likely that several of the Ophiurids and Asteroids which normally have 

 more than 5 arms undergo such fission. LUTKEN ((Efvers. Uansk. vid. Selsk. 

 Fiirh., 1872, pp. 108158 : tr. Ann. and Mag. N. H., 1873, S. 4, xn. pp. 323 and 391) 

 gave an account of this phenomenon. Ophiothcla isidicola (Formosa) generally 

 has 6 arms, rarely equal, usually 3 large opposite to 3 small; specimens common 

 with only 3 arms, with appearance as if corresponding half-disc cut off. There 

 can be no doubt that the animal divides and that the other 3 arms are renewed. 

 The same phenomenon has been seen in other small 6-armed Ophiurids, especially 

 of genus Ophiactis, but Liitkeu never saw any trace of it in any normally 5-rayed 

 species of the genus. There are indications that the division occurs once when the 

 animal is very small and again when it is adult or nearly so. In Ophiocomu pumila 

 the small specimens have 6 arms, while the adults have 5. Probably therefore 

 division only occurs in the young, the last division being followed by the pro- 

 duction of 1 or 2 arms instead of 2 or 3. 



Division is probably not a usual occurrence even in Ophiurids having more than 

 5 arms. Ophiacantha anomala has normally 6 arms, and 0. vivipara has 78, 

 but no such appearances are known in them. 



Similarly there is evidence [figs, given] that certain Asteroids having normally 

 more than 5 arms viz. Asterias problema Stp. [ = Stichaster attains], A. tennitpina 

 &c. undergo fission ; but there is no reason for believing that other many-armed 

 Asteroids divide. The Solasters have many rays, Asterias polaris has 6, but no 

 signs of division are seen in them. 



An account is also given of the comet-like specimens of Ophidiaster crilrarhis, 

 occasionally found, having one long arm, at the adoral end of which are present 

 4 or 5 arms as mere tubercles or as half-grown structures. This phenomenon is 

 well known in Linckia multiflora, in which doubtless the separate arms may break 

 off, each reproducing complete disc and arms. [See also as to Stichaster albulns, 

 Asterina wega, &c., CUENOT, L., Arch. zool. exp., V. bis, 187990, p. 128; and as 

 to Linckia, SARASIN, Ergeb. naturw. Forsch. auf Ceylon, 1888, i. Hft. 2.] 



B. 28 



