32 



above the maxilliped a single pseudobranchia is found. First pair of thoracic legs much shorter 

 than third maxillipeds, terminating (PI. Ill, fig. 2 /) in a quite short, but not rudimentary chela, 

 and its fingers have a kind of brush near the end; sixth joint near the base and the preceding 

 joint near the end each with a close row of stiff, short setae nearly as those found in Sergestes. 

 Second pair of legs considerably longer than the first and much shorter than the third pair; 

 second and third pairs slender and terminating in a very short chela agreeing with that of first 

 pair; all three pairs with seven joints, but without epipod or exopod. Fourth and fifth pairs 

 of legs wanting, but in the male a couple ot very thick protuberances are found, marked off 

 from the trunk by a suture or rather an articulation, directed mainly forwards and answering 

 to the coxae of a pair of legs, in all probability fifth pair (as already interpreted by Kishinoye) ; 

 these protuberances, which afford specific differences, are on the following pages described as 

 the genital coxae {gc. on PI. Ill, fig. 2;/). Above the thoracic legs four pleurobranchiae are 

 found, situated respectively above the three legs and above the place were the fourth leg, 

 which is wanting, would be placed, but I have found no rudimentary lamellar branchia behind 

 any of the four pseudobranchiae. 



The petasma is much reduced as compared with those in Sergestes and Sicyonella\ pars 

 externa has no processus uncifer, pars astringens is frequently wanting, and the distal portion 

 of pars media has few ramifications, in most species only a processus ventralis and a capitulum 

 without special lobes. 



Remarks. — The genus Acetes differs from Sergestes in several characters: the maxil- 

 lulae and the first maxillipeds without palp, the maxillae with undivided lobe, first pair of legs 

 with a short chela as the two following pairs, fourth and fifth pairs of legs wanting excepting 

 the coxae of (probably) fifth pair in the male, finally only five pleurobranchiae above third pair 

 of maxillipeds and the thoracic legs. — In the absence of two pairs of thoracic legs Acetes 

 agrees with Lucifer, but otherwise it is far removed from this peculiar genus and related to 

 Sergestes and Sicyonella. 



The species of Acetes are very similar in general aspect, and all are rather small animals. 

 The eyes are nearly as large in the female as in the male. Specific characters found in both 

 sexes are few, viz. the number of denticles — one or two — on the rostral crest, the size of 

 the eyes, the relative length of the proximal thickened part of the upper flagellum of the 

 antennulae, the length of the ciliated portion of the outer margin of the exopod of the uropods 

 as compared with the whole margin, finally to some degree the second joint in third pair of 

 legs. The males show excellent specific characters in the relative length of third joint of the 

 antennulae, in the joints of the lower antennular flagellum and especially in the structure of its 

 clasping organ, finally in the structure of the petasma. In the females the ventral area at and 

 behind the base of last pair of legs affords most useful characters. The females are on the 

 whole somewhat or even considerably larger than the males. 



The genus Acetes was established by H. Milne-Edwards in 1830 (Ann. Sci. Natur. 

 T. XIX, p. 350 ; PI. XI, figs. I — 9) on a species captured in the Ganges estuary and named 

 A. indicus. His representation is fairly good, but the description of the form is of course now 

 unsatisfactory. Fortunately his fig. 9, exhibiting the caudal fan, shows that the ciliated part of 



