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



Crustacea, with which Milne-Edwards says the species of this genus establish a connection. 

 Attached to the mastigobranchial plate is a well-developed but small podobranchial 

 plume. Between this and the next somite, deep in the interstitial groove, is a small 

 foliaceous plate of a low degree of structure ; on the somite that carries the second pair of 

 gnathopoda is a larger branchial plume, which has no connection with the gnathopod 

 or its membranous articulation ; this is followed by an interstitial foliaceous plate on 

 the three following somites ; on the last somite the foliaceous plate is wanting, but a 

 second branchial plume is present besides the normal one, while the last somite has 

 neither plate nor plume. 



The arrangement, 1 as seen in Sergestes japonicus, is as follows : — 



Pleurobranchise, . . . ... fl fl fl fl 2 



Artlirobranchias, . . 



Podobranehise, . . . 1 



Mastigobranchise, . . . 1 



h i k 1 in n o 



111 Sergestes hroyeri, however, there are two branchiae attached to the penultimate 

 somite, two to the antepenultimate, an interstitial foliaceous plate between the next 

 three somites, and one branchial plume but no interstitial leaf between the next. This 

 may be better understood by the following tabular arrangement, by which it may be 

 seen that one foliaceous plate has been replaced by a branchial plume. 



Pleurobranchise, . . . ... fl fl fl 2 2 



Arthrobranchiae, . . 



Podobranchia?, . . . 1 



Mastigobranchue, . . . 1 



li i k 1 in n o 



In Sergestes prehensilis this feature is carried a degree still further, inasmuch as 

 the interstitial foliaceous plates exhibit signs of breaking up into branchial plumes, too 

 unimportant to be pronounced plumes, and too much differentiated to be called foliaceous 

 plates. 



Thus it appears that these interstitial foliaceous plates are rudimentary branchial 



Geographical Distribution. — The species of this genus are chiefly oceanic, and have 

 been taken all over the globe, in the Arctic as well as in the Tropical seas. 



1 Claus in bis Morphologie der Crustaceen (p. 52) has tabulated the branchiae of Sergestes in accordance with 

 the table given on this page for Sergestes japonicus, with the exception that he makes the foliaceous plates in his 

 diagram (Taf. iii. figs. 31, 2c, 3c, Ac, be), which he has figured as large as those in the genus Petalidium, as well as 

 the antepenultimate (6c), and ultimate (7c) plumes, to correspond with the arthrobranchia?, and the other branchial 

 plumes (c') to homologise with the pleurobranchia; ; but, according to my observation, none of the branchire are attached 

 to the membranous tissue that unites the coxa; of the pereiopoda to the somites of the pereion. 



