Growth-changes in Brittle Stars. 97 



acterized by its color (above green, more or less variegated with whitish 

 or yellowish; below whitish or yellowish), its rather short arms (5 to 6 times 

 the disk-diameter), long and large radial shields, spinelets on disk especially 

 around the margin, 5 or 6 arm-spines, and 2 (sometimes i or 3) oral papillae. 

 The genital slits are well-developed and bursae seem to be present, so that 

 in these particulars as well as in the presence of conspicuous radial shields, 

 savignyi is easily distinguished from the European Ophiactis virens. Like 

 virens, however, savigjiyi is characterized by the remarkable extent to which 

 schizogony is carried as a means of reproduction. The process is apparently 

 identical in the two species and has been very fully described by Simroth 

 (1876), whose observations were based on the European form. The process 

 seems to begin, in savignyi at least, almost as soon as the adult form is 

 assumed, for the youngest specimen found has already divided at least once 

 (pi. I, fig. 2). 



No evidence was secured on the important points of egg-laying and 

 larval form in Ophiactis. None of the adults contained either embryos or 

 eggs large enough to be visible under a lens. The impression gained from 

 the examination of the living material was that the eggs are laid and fertili- 

 zation and development take place outside the body of the mother. There 

 was no indication whatever that the species is viviparous. It was not 

 possible to decide whether egg-laying was completed for the season by 

 March first, but the appearance of the adults and the great scarcity of very 

 young specimens led me to the opinion that breeding was over. 



My observations confirm Simroth in his statements that there is no special 

 plane of division in schizogony, and that the process shows more or less indi- 

 vidual diversity. Thus in fifteen young specimens selected at random, show- 

 ing evidence that division had recently occurred, fourteen had 3 arms, the 

 other only 2 ; the latter had 3 jaws and 3 oral shields, however. Of the four- 

 teen 3-armed specimens three have 4 jaws and four have only 2. The plane 

 of division, which passes through two opposite interradii, does not then divide 

 a jaw, but passes sometimes on one side and sometimes on the other. Only 

 about half the time does it divide the animal into perfectly complemental 

 halves. The process of healing and the simultaneous rapid growth of the 

 water-vascular system have been well described by Simroth. Almost, if 

 not actually, before the circular canal is completely healed buds which will 

 give rise to new radial canals have pushed out, one at each side of the 

 healing margin of the now semi-circular disk, and before there is any 

 external evidence of the new half -disk the buds of two new arms, one at 

 each side, become visible. Before their growth has proceeded very far, 

 however, a third arm appears between them, associated with the third 

 outgrowth of the now completely healed circular canal. The failure of this 

 third radial canal to start development probably accounts for some of the 

 5-armed individuals met with, but as the plane of division occasionally 

 leaves 4 arms on one disk-half and only 2 on the other, it is evident that 

 these 2-armed halves will form 5-armed adults even with a normal develop- 



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