CRANGON VULGABIS. 147 



parison of figs. 10 and 11 shows, as was mentioned above, a 

 considerable difference in the size of the embryo, the older 

 being considerably the smaller of the two, the original 

 dimensions not being regained until the stage just past. 

 Mayer ('77), calls attention to a similar state of affairs 

 in Eupagurus, while Ishikawa's figures show that the 

 same occurs in Atyephyra. In Astacus, according to 

 measurements of Reichenbach's plates, there is a similar 

 contraction of the germinal area, though not to so great an 

 extent as in the other forms mentioned. So far as I now 

 recall, this circumstance is not readily paralleled in the 

 animal kingdom, nor is it easy to explain. Indeed, I can 

 think of but one interpretation to be placed upon it and 

 that is not over satisfactory. It is as follows : almost all 

 decapods now leave the egg and begin a free life in a com- 

 paratively advanced condition, but the evidence presented 

 by Lucifer, Penseus, etc., shows that their ancestors be- 

 gan their free life when much more immature 1 , or at least 

 when in a condition far less like that of existing adults 

 than is the newly hatched embryo of to-day. There is 

 evidence that this early crustacean had an egg with com- 

 paratively little food yolk; indeed, this element seems to 

 have been introduced at a comparatively recent date. For 

 such an embryo it would be a great advantage to begin its 

 free life with only those organs necessary to its existence 

 and hence the more rapidly the whole egg was converted 

 into the germ the better for the individual and hence for 

 the race. The more direct the development within the 



1 As will appear in the sequel, I do not give the nanplius that extreme phyloge- 

 netic importance which many do. I regard it as an introduced, adaptive, larval 

 condition which, of course, has become hereditary, and marks a connection be- 

 tween all Crustacea, but which must not be regarded as representing the adult 

 condition of any ancestor. The arguments are too many against such a view. A 

 full discussion of this and allied points is reserved for the concluding sections of 

 my studies of Crangon. 



ESSEX INST. BULLETIN, VOL. XVIlt. 19 



