440 



MEKISTIC VARIATION. 



[PART i. 



seen one with X. Individuals with 4 arms occur, but are much 

 less common tli.-m those with 6. I have seen Asterina gibbosa 

 with 4 ray>, and a -prrimeii (Scilly) given me by Mr S. F. Harmer 

 has ii rays, of which 2 are a little nearer together than the others 

 (su^.'-tin^ division of ,-i ray). Mr K. \V. MaeBridr tells me that 

 In- has sc.-ii st -\i-ral 6-rayed specimens of this species. Mr E. 

 .). Bl'-s kindly tells me that he dredged a 4-rayed Porania 

 pulvillus in the Clyde estuary. There appeared to be no trace 

 of a tilth ray and the specimen was as nearly as possible sym- 

 metric:! I. 



Tin 1 following cases exhibit special points. 



671. Asterias glacialis : specimen with 8 rays possessed 3 madre- 

 poritfs. COUCH, J., C/mrleswort/t's Mag. of N. H., 1840, iv. p. 34. 



072. Asterias rubens : 6-rayed specimens frequent at Wimereux. In 

 several of these there are two sand-canals terminating at a common 

 madreporite. GIARD, A., Comptes rendus, 1877, p. 973; cp. id. C. R. 

 sec. biol., 1888, p. 275. 



G73. /''irfiiif il'u-ixiun of an arm is fairly common in Asteroids, but less 

 common I believe than the total variation in number, though I know 

 no statistics on this point. For a figure of Asterias (Hippasterias) 

 equestris L. with a bifid arm, presenting no appearance as of regener- 

 ation see TIKDKMANX, Zdtxchr. f. Phys., 1831, iv. p. 123, Plate 1. 

 The two following are peculiar cases. 



6 / 4. Cribrella oculata : one of the arms bearing a branch, not as a 

 radius, but about (in dried specimen) at right angles to the normal arm, 

 the property of Prof. C. STEWART, who kindly shewed it to me. 



675. Porania pulvillus, Cray (a Starfish): Specimen 5 cm. in diame- 

 ter, having five short rays. The ray opposite the madreporite u-Jien 

 viewed from the, aboral surface is seen to be distinctly bifurcated at 



rara&sgsg 



w 



^:ll ^ 



Kx^'ffl 



, y& 





Fio. 134. Pitrniii'i jiuli-illnx, No. 675, having the arm opposite the madreporite 

 abnormally divided as shewn at x and y. (From a sketch kindly sent by Prof. 



I ll UI'MAN.) 



