MRTAMORPHOSTS OF CORYSTES CASSIVELAUNUS. 467 



Tlie two pairs of maxillipedes Lave each a two-jointed 

 exopodite bearing distally four long ciliated setae. 



Behind the first and second maxillipedes there are already 

 developed rudiments of the six remaining pairs of thoracic 

 legs. The first pair^ or third maxillipedes, are longer than 

 the rest, and bent forwards between the second maxillipedes. 

 The fourth pair is covered by the third, and hence is not 

 visible without dissection (see fig. 1). 



As breeding females of Corystes are not easy to obtain, 

 and I have consequently not been able to hatch the zosea 

 from the egg, it is possible that an earlier stage remains to 

 be discovered. Still, the early development of the posterior 

 thoracic limbs is not uncommon among Brachyurous 

 zoaeas. In many forms the third maxillipede is already 

 marked out in the first zofea, and in some all the thoracic 

 appendages are visible on hatching. This is the case in 

 P o r t u n u s p u b e r, and more especially in I n a c h u s d o r s e t - 

 tensis, where even the pleopods are distinctly traceable. 



The abdomen, in the first stage of Corystes, consists as 

 usual of five distinct segments, the second bearing a for- 

 wardly curved process on either side. The second, third, 

 fourth, and fifth segments each bear a short hair on their 

 posterior dorsal edge on either side of the middle line. 



The telson (fig. 2), with which the sixth segment is united, 

 has the usual forked shape, and bears three strong setee on 

 the inner surface of either fork. Each seta is minutely 

 ciliated, the first, however, bearing several much longer cilia 

 about the middle of its length. There is only one external 

 spine (the sixth of Mayer's nomenclature). The spine 

 formula is therefore 5 + 5 instead of the normal 7 + 7. 



Second Stage (fig. 8). — The following measurements 

 are the average of the first ten specimens of a number 

 measured, and the limits of variation in those specimens. 



