REPORT ON THE CRUSTACEA MACRURA. 345 



The chief differences that separate this species from Gennadas parvus are the 

 pointed character of the prominent secondary eye-stalk, the less prominent lobe-like 

 condition of the upper surface of the third joint of the first pair of antennae, the greater 

 amount of squamiform expansion of the meral and ischial joints of the gnathopoda, the 

 presence of a short, stout tooth on the outer margin of the dactylos, midway between the 

 base and the apex, and of a fine but stiff spine at the extremity, and probably the greater 

 relative length of the telson as compared with the outer plates of the rhipidura. 



This specimen was taken along with Ilcdiporus and Acanthcpltyra. 



Family Seegesiid^;. 



In this family the diminishing value of the two posterior pairs of pereiopoda is very 

 conspicuous, both becoming small, enfeebled, and in some instances rudimentary, as in 

 Sergestes. In Acetes the penultimate pair is reduced and the posterior is wanting, while 

 in Leucifer they are both entirely absent. The branchial system has also become of 

 diminished importance. The mastigobranchise are all wanting except a rudimentary 

 discoid plate at the base of the first pair of gnathopoda ; so also are the podobranchial 

 plumes, except a small plume attached to the first gnathopod. The arthrobranchias are 

 all wanting in the genus Sergestes, but are retained in Petalidium, except on the last two 

 pairs of pereiopoda, and the pleurobranchise are retained in all the genera in a more or less 

 modified form. 



Subfamily Sergestin^e. 



This subfamily comprises those genera that possess branchiae of an impoverished 

 character. 



The mastigobranchias and podobranchiaa are wanting, excepting a rudimentary 

 pair, attached to the first pair of gnathopoda. The ovum of this subfamily has 

 never been observed, but specimens of an early Zoea stage, believed to belong to this 

 subfamily, have been taken and watched, and I therefore think it probable that the ova 

 are deposited and hatched in the sea, and not carried by the animal. The brephalos is 

 stated to be in the Nauplius form but as yet this is only hypothetical, since its connection 

 with the parent has never been observed. The Zoea form to which Anton Dohrn : has 

 given the name of Elaphocaris, is the earliest known stage according to the results of 

 the Challenger exploration. But Professor Brooks states 2 that " the metamorphosis of 

 Sergestes is more like that of Leucifer than is the case with any known Crustacean except 

 Acetes, but our knowledge of Sergestes is incomplete." 



In this subfamily there are several genera, Petalidium, Sergestes, Acetes, and Sciacarus. 



1 Zeitschr.f. miss. Zool, Bd. xx. p. 607, 1870. - Phil. Trans., p. Ill, pi. i., 1882. 



(ZOOL. CHALL. EXP. PART LII. — 18S6.) Fi'f 44 



