No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 137 



Whether or not these views are correct later observations 

 will determine, but they permit of a reduction of the develop- 

 mental phenomena of the Crustacea to a common type, and in- 

 dicate how the various modifications described have been 

 brought about. 



Part IV. — Notes on the Development of Certain 



Organs. 



The observations recorded in this portion of the paper are 

 fragmentary, and consist simply of a few facts which have been 

 incidentally noticed concerning the development of certain or- 

 gans, and which do not fall strictly within the limits of this 

 paper as originally laid out. 



The development of the nervous system I hope some time in 

 the future to work out thoroughly, and shall merely notice here 

 two points concerning it. The first of these is the occurrence 

 of a teloblastic mode of growth of the nerve ganglia in the 

 Isopods, of the same character as that described by Bergh ('93) 

 as occurring in Mjsis, and similar to what Wheeler ('91) has 

 observed in insect embryos. 



The second point is the occurrence in the embryos of all the 

 Isopods I have studied of a pair of ganglia, unaccompanied by 

 corresponding limbs, and lying in front of the antennular gan- 

 glia. These are shown in Figs. 69 and 70, of which Fig. 69 

 represents an embryo of CymotJioa, and Fig. 70 one oi Jaera in 

 a slightly later stage of development. From these figures the 

 following arrangement of the ganglia can be seen. Anteriorly 

 on either side are the large optic ganglia {op), and nearer the 

 median line two elongate oval masses, which are to be regarded 

 as the cerebral ganglia proper (cc), the optic ganglia being 

 phylogenetically probably a secondary differentiation from 

 these. Behind and externally to the cerebral ganglia is to be 

 seen on either side a thickened mass of tissue {G), which is the 

 ganglion to which special attention is directed here, and behind 

 it lies the antennular ganglion {ciG), situated in front of the 

 mouth invagination {st), the other two naupliar ganglia {at G 

 and mG) lying behind this structure. 



