62 W. K. BROOKS AND E. B. WILSON. 



the embryonic skin prevents the extension of the locomotor 

 hairs. 



In Figure 1, the rudimentary third maxilliped is shown behind 

 the base of the second. 



In Plate VII, Figure 4, the third maxilliped, c, is shown, more 

 highly magnified, lying in the same series with the bases, a and 6, 

 of the first and second. A fourth appendage, no doubt the first 

 pereiopod, is also represented at this stage by a bud or rudiment, 

 d, and the appendages, 6, c, and d, are furnished with little buds, 

 which would seem to be rudimentary gills. After the moult we were 

 not able to detect either the appendage, d, or the gill-like processes. 



After the embryonic skin is moulted, the locomotor hairs of the 

 first and second maxillipeds lengthen and these appendages become 

 functional, while the third pair remain rudimentary. Figure 6, 

 Plate VII, shows the first and second maxillipeds soon after the 

 moult, and hardly calls for explanation. 



The embryonic skin conforms closely to the surface of the ab- 

 domen and telson, although it appears to have no trace of a 

 division into somites. 



Figure 7 of Plate VI shows one-half of the telson of Figure 1 

 before the embryonic skin is shed. A comparison with Figure 6, 

 T, will show that the great difference which has been pointed out 

 by Faxon and others between the telson of the embryonic skin and 

 that of the zoea in the ordinary crab, does not occur in Porcellana, 

 but that the two are here nearly alike. 



The five pairs of long swimming hairs of the zoea are, before the 

 moult, about half invaginated, and the extended portion, Plate VI, 

 Figure 8, is finely plumose. The hairs of the embryonic cuticle 

 are much stouter, and their edges are not plumose, but they agree 

 with those of the zoea, in number and arrangement. 



The outer hair, or marginal spine of the telson, has the same 

 appearance before the moult that it has afterwards. 



EXPLANATION OF THE FIGURES. 



PLATE VI. 



FIGURE 1. Zoea immediately after its escape from the egg, seen from 

 the left side. From a drawing by W. K. Brooks. 



