I J2 W. K. FISHER. 



markedly from the adult 4 that they have been described as of a 

 different genus. I have found this condition in three species: 

 Sderasterias euplecta (Fisher), 5. Jieterop&s, new species, and S. 

 alexandri (Ludwig). The young of the last was described by 

 Ludwig as Hydrasterias diomedece, while Hydrasterias richardi 

 (Perrier) is the fissiparous phase of an unknown adult. 



In this stage the sea stars actively divide by splitting into equal 

 halves. The number of these divisions is not known. In one 

 instance, in 5. euplecta, a division has taken place nearly at right 

 angles to the plane of the prior fission. Since adult Sderasterias 

 are almost invariably five-rayed and the majority of the young 

 are six-rayed, an arm is lost somewhere in the process probably 

 at the last division. These dividing young range in size from 

 R 8 mm. 5 to R 20 mm., the latter being unusually large. R 15 

 mm. is nearer the normal maximum size of fissiparous individuals. 



The species of Sderasterias live in deep water, usually among 

 rocks. All the specimens of fissiparous young which I have seen 

 were taken by means of hempen tangles. The data for the 

 following notes are admittedly incomplete. There is every 

 reason to believe, however, that many years will pass before more 

 material is forthcoming. 



Of the thirty-six young of S. euplecta examined, nine, or one 

 fourth, have five rays. None of these show signs of fissiparity 

 although two have lost individual rays. All of the twenty-six 

 fissiparous specimens have six rays. Of these six rays, three are 

 usually smaller and represent the regenerating half. 



In S. heteroptzs, fourteen of the young have six rays and only 

 one has five rays. Twelve out of these fourteen six-rayed 

 specimens have four madreporites symmetrically placed, with two 

 on either side of the plane of fission (the two opposite interradii 

 through which the disk splits being without them). Thus each 

 half, after fission, has 2 madreporites one on either side of the 



4 It is not essential to introduce data to prove that these specimens are really 

 young Sderasterias, although there is ample evidence. In the fissiparous stage the 

 crossed pedicellariae (which are like those of the adult) are not concentrated in 

 circumspinal wreaths as in the adult, but are scattered between the spinelets. 

 The latter are short, uniform, and two to four on each median radial plate and two 

 or three on each superomarginal plate. In the adult only each alternate plate of 

 the two series mentioned carries a single spine. The extra spines are absorbed. 



6 R is the distance between the center of disk and tip of ray the major radius. 



