Division in the Echinodermata. 329 



What we observe in Asterias tenuispina probably occurs 

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

 A. microdiscus, Stimps.f (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 problema 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 = 5-1 9 millims.) : in general the five 

 arms 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 J. 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, d,f, 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 found 5+ 4, 

 4+4, and 2+5 arms (Proc. Bost. Soc. Nat. Hist. viii. p. 262). 



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

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



\ The appearance of five arms arises sometimes from the union of two 

 arms; the double ambulacrum explains this apparent reduction of the 

 arms. 



