REPORT ON THE ISOPODA. 17 



lu the females there is no trace whatever of these penial filaments. 



2. Under this head I may briefly review certain outward ditferences in male and 

 female individuals which are peculiar to one or more species, and of which a fuller 

 description is given below. 



In Serolis convexa (PI. VI. fig. 10) and Serolis gaudichaudii the second pair of 

 thoracic appendages in the male are furnished with a tuft of pinnate hairs upon the 

 inner side of the antepenultimate joint, and in the male of the former species, at any rate, 

 the sterna of certain of the thoracic segments are provided with patches of similar hairs. 



In Serolis septemcarinata the epiraera of the males bear a row of five or six elongated 

 ridge-like tentacles on the under surface which are entirely wanting in the female. 



The females of Serolis nexera are distinguished from the males by the fiir greater 

 development of the frontal "sense organ"^ {cf. PL V. figs. 1, 3), and the epimera them- 

 selves show certain difi'erences in the two sexes, being considerably wider in the male, 

 where the anterior margin from the point of junction with the lateral angle of the 

 cephalic shield slopes gradually backwards and outwards to the termination of the 

 epimeron ; in the females the anterior margin passes at first directly backwards almost 

 parallel to the longitudinal axis of the body, and then slopes outwards so that in this 

 sex the anterior epimera look as if a piece had been cut out. 



Another sexual diff"erence is found in Serolis gracilis and Serolis convexa, and is men- 

 tioned by Audouin and Milne-Edwards as occurring in Serolis gaudichaudii^^ In these 

 species the terminal thoracic appendages of the males are beset with fine hairs, which are 

 similar in shape to the hairs upon the second thoracic appendages of the males of Serolis 

 convexa and Serolis gaudichaudii; these same appendages in the females do not difi'er in 

 any way from the preceding thoracic appendages. 



On PL II. fig. 6 is figured a single joint from the filament of the second antenna 

 of a male Se^'olis scliythei ; along the inner margin of the joint are a series of delicate 

 fan-shaped lameUse which I did not succeed in finding in any female example of this 

 species; it is possible, however, that on account of theii- extreme delicacy they may have 

 been overlooked. 



Alimentary Canal. — The alimentary canal of Serolis agrees closely with that of other 

 Isopoda; the point to which I may call special attention is the presence of salivary glands, 

 which I have noticed in Serolis ixtradoxa and Serolis septemcarinata. On account of 



' Grube was tlie first to point out the presence of a transparent oval area on the first epimera wliich he imagined 

 might cover some sense organ ; Dr. Woodward (Geol. May., 1883, p. 21) has compared this structure to certain pores 

 which are to be found in many Trilobites occupying an identical position. I have foimd that this structure exists in 

 nearly all the species of Serolis, generally having the form of a deep and narrow groove surrounded by a specially 

 thickened rim; in SerolU schytlwi and Serolis cornuta the structure is precisely as described by Grube in Serolis paradoxa, 

 and as in that species there is a jiore on the under surface of the epimeron exactly beneath it. In Serolis antardica and 

 others I could discover no trace of it. Concerning the minute anatomy of this " sense organ," I am unfortunately able 

 to say nothing. 



^ Loc. cit., p. 19. 



(ZOOL. CHALL. EXP. PART XXXIII. — 1884.) Kk 3 



