330 



Dr. C. F. Llitken on Spontaneous 



groups of arms occurs in all possible degrees : the smaller 

 grou]) may sometimes scarcely differ from tlic larger ; or it may 

 be reduced to 3 (1, 2) scarcely visible buds (fig. 1, ^). Of the 

 3 (or 4) regenerated arms the middle one (or 2) generally ap- 

 pears after the two outer ones. Numbers such as 4 + 2, or 



Fig. 1. 



Sketches of 9 individuals of Asterias prohlema, all represented of the 

 natural size. In most the madi-eporic plate is indicated. 



2 + 4, or 3 + 2, or 2 + 3 must be regarded as exceptions, as also 

 4 + 3(%. l,Z»,c), or 3 + 4, or4 + 4 (fig. 1, a), or 4 + 5, or 5 + 2, 

 in which the total number of arms exceeds six. Evidently in 

 all cases the smaller group of arms is developed long after the 

 other, and consequently there must have been a period when 

 all these starfishes had only 3 (or exceptionally 2, 4, or 5) 

 arms ; in examining a sufficient number of specimens we find 

 several with 3 arms, in which no trace of the deficient arms 

 can be discovered, and among these some (fig. 1, ^) in which the 

 place where the division probably took place, and where the 

 new arms will be formed, is still open. These specimens with 



3 arms (exceptionally with 2) I have found of all sizes, from 

 3 to 25 millims. or more in radius ; moreover, as the specimens 



