692 THE VOYAGE OF H.M.S. CHALLENGER. 



The pleon is also laterally compressed, and is as deep anteriorly as the carapace, 

 but posteriorly it gradually tapers to the extremity of the telson, corresponding with 

 the line of the dorsal surface. 



The ophthalmopoda are short but free. 



The first pair of antennae terminates in two flagella. 



The second pair of antennae carries a broad foliaceous scaphocerite armed with a 

 tooth on the outer margin, and a long flagellum. 



The mandibles have a psalistoma that is continuous with a molar process, but 

 there is no synaphipod. 



The gnathopoda are short and membranous. 



The first two pairs of pereiopoda are short, submembranous, and chelate, with the 

 extremity of the pollex and dactylos furnished with long hairs. The third pair is 

 simple, long, and very robust. The fourth and fifth are shorter and robust. 



The pleopoda are short and foliaceous. The rhipidura is well developed and 

 robust, the telson being shorter than the lateral plates. 



The description of the form to which Randall has given the name of Atyoida so 

 closely corresponds with Atya that it is difficult to see how it can be retained as a 

 separate genus. It is undoubtedly smaller in size, and has the posterior three pairs of 

 pereiopoda comparatively more feeble in character and proportionally smaller and sub- 

 equal in size. 



Dana, in his great work, expresses the opinion that " among the species of Atya 

 there is a very great difference as to the relative size of the third and following pairs of 

 legs ; and it seems possible that the transition may be such as to render it unnecessary 

 to sustain the genus Atyoida." 



A close examination of the structural details confirms this opinion of Dana, and 

 therefore place Atyoida in this Report under Atya, as at present it appears to me that 

 the weight of our experience leads to the belief that the two named forms are but 

 different species of the same genus. 



Development. — Among the numerous specimens of Atya {Atyoida) bisulcata, 

 procured in the market at Honolulu, there were several females carrying ova, of a 

 long ovate form (PL CXXII. fig. 2ov), and of these one or two specimens had the 

 embryo so far advanced in development that on rupturing the egg-case I was able to 

 determine the form of the brephalos. 



This is in an advanced Zoea stage (PI. CXXII. fig. 2), corresponding with that of the 

 marine forms of the normal Phyllobranchiata, differing from that of Crangon only in 

 the absence of a tooth on the third somite of the pleon, and considerably resembling 

 that of Atyheus, from which it differs in not having the ophthalmopoda detached 

 from the frontal margin of the carapace, but large and apparently continuous 

 with it. This, how r ever, is a consequence of its embryonic condition, and probably 



