THE DEVELOPMENT OF OPUIOTHRIX FEAGILIS. 575 



tozoa, yet they may be prematurely produced by the presence 

 of these, or possibly by the mechanical irritation of shaking 

 the ovary, and that in these cases the resulting development 

 is abnormal. This abnormality must be due therefore to a 

 difference in the chemical constitution of the egg, and the 

 fact that one abnormal feature appears to be normal in 

 another species seems to be an indication that very slight 

 chemical alterations in the constitution of the fertilised ess: 



DO 



may produce marked structural alterations in the organism 

 which results from it, and that these alterations may be 

 inheritable. Here, perhaps, is to be found the origin of those 

 mutations which, according to recent speculation, constitute 

 the raw material with which evolution woi-ks. But for the 

 solution of this problem animals must be selected, which are 

 easier to breed and in Avhich the infant mortality is less than 

 in Echinoderras. 



The Development of the Full-grown Larva. 



We left the consideration of the normal development at 

 the point where the coelom had arisen from the apex of the 

 archenterou as a bilobed vesicle, and the vacuolated crest had 

 diminished in size, whilst the postero-lateral arms were making 

 their appearance. By the end of the second day the crest has 

 entirely disappeared, whilst the postero-lateral arms have 

 grown so much that the organism takes on the form of a V 

 with a thickened point. The ccelom separates as a single 

 vesicle from the archenterou, while the rest of the archenteron 

 constitutes the definite gut (PI. 33, fig. 29). Soon afterwards 

 the single coelomic rudiment divides into right and left halves, 

 and the stomodseum makes its appearance as a wide, shallow 

 pit just ventral to the place where the crest disappeared (PI. 31, 

 fig. G). In fig. 30 the stomodasnm is shown in the act of open- 

 ing into the gut, and from a comparison of this specimen with 

 older specimens, sections of wliicli are shown in figs. 31 and 

 32, the conclusion is arrived at that, as in Echinus escu- 

 lent us, the V-shaped adoral ciliated band which projects 



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