No. I.] EMBRYOLOGY OF THE ISOPOD CRUSTACEA. 1 39 



and furthermore it is to be noted that they do not appear con- 

 temporaneously with the limbs, making their appearance long 

 after the limbs are readily distinguishable. 



Finally I have introduced two figures {Figs. 71, 72) repre- 

 senting late stages in the development oi/aera, which illustrate 

 a point of some significance in connection with the phylogeny 

 of the Isopods. In the adults of these forms there is no indi- 

 cation of the presence of a carapace, but in Fig. 71 a fold 

 {car) is clearly seen which extends backward to behind the first 

 pereiopod {t/P) and represents a rudimentary carapace. In the 

 stage represented in Fig. 72 this fold covers relatively a smaller 

 extent of the body, its posterior margin being on a level with 

 the interval between the maxilliped and the first pereiopod. I 

 think there can be no question but that this fold represents a 

 rudimentary carapace, and points to the derivation of the Isopods 

 from ancestors possessing a more or less perfectly developed 

 carapace fold. Whether these ancestors are most accurately 

 represented to-day by the Schizopods, as some have maintained, 

 or by the Cumacea remains to be seen, contributions on the 

 development of the latter group being much required, the 

 observations of Blanc ('85) leaving many points in connection 

 with their development unsettled. 



Marine Biological Laboratory, 

 Woods Holl, Mass., 

 August, 1894. 



