Division in On- EchinodenmoJa. 



333 



This singular phenomenon was briefly mentioned by M. 

 Steenstrup ; and MM. Hackel and Von Martens subsequently 

 published observations made by them upon it in Linckia mul~ 

 tifora. I have rarely observed it in that species, although I 

 have examined a great number of specimens ; but I have 

 frequently seen it in Linckia omithopus and Ophidiaster cri- 

 brarius. In both species, as in the flssiparous species of 

 Asterias above mentioned, there are in general two madreporic 

 plates ; and they are more or less frequently provided with 

 more than five arms. In examining a great quantity of spe- 

 cimens of one of these species, we find a very variable number 

 of arms (4, 5, 6, 7) ; and the proportions of these arms are very 

 different — for example, 3 long alternating with 3 short ones, 

 4 long and 2 short, or inversely 2 long and 4 short ; from time 

 to time also we meet with the " comet-form " — i.e. one long and 



Fiff. 2. 



r w 



Sketches of 4 individuals of Ophidiaster cribmrius, represented of the 

 natural size. 



thick arm, and 3, 4, 5, or 6 comparatively short arms. The 

 hypothesis which attributes this form to the production of the 

 small arms after the large one, is confirmed by the circum- 

 stance that other specimens in which these little arms are still 

 less occur in various degrees of development ; and, lastly, 

 there are specimens in which they are in the state of mere 

 buds — until we find them with a single arm, either closed at its 

 adoral extremity or still presenting traces of the aperture 

 through which it had communicated with the disk of the 

 animal (see the outlines in fig. 2). From this it seems very 

 clear that the regenerative faculty in these animals is so great 

 that an isolated arm, without any portion of the disk, enjoys 

 the power of regenerating a complete set of arms, with the 

 disk, mouth, &c. ; and as we cannot suppose that one arm will 

 be more favoured in this respect than the others, it follows 

 Ann. & Mag. N. II. Ser. 4. Vol xii. 23 



