Division in the EcMnodermato,. 329 



Wliat we observe in Asterias tenuispina probaLly occurs 

 also in the allied species, A. acutispina^ Stimpson* (Japan), 

 A. onicrodiscus^ Stimps.t (Bonin islands), and A. muricata^ 

 Verr. (New Zealand) ; but the specimens at my command are 

 too few to enable me to assert that spontaneous division takes 

 place in these species ; it is, however, very probable. It is 

 the same with A. atlantica, Verr. (Bermudas, Brazil), if it 

 differs from A. tenuispina. Mr. Verrill mentions one speci- 

 men of it with seven large unequal arms, and one with eight 

 arms, four of which were smaller than the others. 



Of Asterias prohlema I have examined several hundred 

 specimens obtained from Greenland by Prof. Steenstrup, and 

 noted the characters of about half of them. It is extremely 

 rare to meet with five-armed specimens of this species. Out 

 of 136 I have only found 7 such (or about 1 in 20) ; their size 

 is very variable (radius = 5-19 raillims.) : in general the five 

 arras are of the same length, and then it is possible that this 

 number five may be original ; less frequently there are two or 

 three a little shorter, probably due either to an irregular divi- 

 sion of a six-armed individual forming one with four arms, 

 and another with two arms which has become a five-armed 

 individual by regeneration, or to an individual with three arms 

 having regenerated only two arms instead of three, the third 

 having been aborted. If we carefully examine a specimen 

 which apparently has only five or four arms, we shall fre- 

 quently find at one of the angles of the arms the germs of two 

 new arms in the form of minute buds ; so that the small num- 

 ber of arms is in this case only provisional |. It is rare, 

 moreover, to find specimens with six or seven arms in which 

 the arms are either equal or approximately equal, without our 

 being able to recognize a fixed law in the slight difference 

 which they present (fig. 1, e, p. 330) : I have found this only 

 in 12 of the specimens mentioned above ; and these equal- 

 armed specimens measured from 5 to 41 millims. in radius. 

 The great majority (fig. 1, t?,/, i) are furnished with six arms, 

 three of which, on one side, are shorter and in all respects less 

 developed than the others ; and this difference between the two 



shores, in deep water and in sheltered places, we find much larger and 

 more regular specimens." 



* In 4 specimens belonging to this species, Mr. Stimpson fomid 5-|-4, 

 4+4, and 2+6 arms (Proc. Bost. Soc. Nat. Hist. viii. p. 2G2). 



t Sent under this name by the Smithsonian Institution. I do not find 

 it described in Mr. Stimpson's work mentioned above. 



I The appearance of five arms arises sometimes from the unicn of two 

 arms; the double ambulacrum explains this apparent reduction of the 

 arms. 



