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



this specimen bad been kept in alcohol in a tube instead of being mounted upon a slide, 

 the thoracic appendages are very much damaged, and in no one instance complete; 

 the basal joints, however, entirely resemble those of Ischnosoma bacillus, so that there is 

 little reason to doubt that the appendages were not widely different. 



The present species being, unlike the last, a male, 1 am able to describe the modifica- 

 tions of the abdominal appendages in this sex, which has not yet been done fir any 

 species of the genus Ischnosoma. I am therefore anxious to discuss them as accurately 

 as possible, seeing that in all probability the following description will be of the genus 

 and not of the species only. 



The first pair of abdominal appendages are shown in fig. 11 of PI. VI. 1 from the 

 inferior surface ; these appendages are somewhat narrow and do not of course entirely 

 roof in the cavity of the abdomen, the covering of which is completed by part of the 

 next pair of appendages. At the upper extremity of the operculum are two minute 

 calcified plates which seem to me to represent respectively the basipodite and endopodite 

 of the limb, the outermost piece being the former. The operculum itself, comprising the 

 two exopodites which fit close together but are separated by a distinct groove, is convex 

 on its outer surface ; the inner border where each piece comes into contact with its 

 fellow is straight, the outer border curved ; the lower margin is slightly incised and on 

 the under surface as shown in the figure there is a transversely elongated aperture. 



The second pair of appendages (fig. 10) are modified into a copulatory organ, which 

 is similar in its general appearance to the same appendage in other Asellidse (cf. 

 Acanthomunna, PI. XII. fig. 13) ; the penial filament, which is swollen at its base and 

 furnished with an aperture, no doubt serving to conduct the semen, is of very great length, 

 and is attached by a short and narrow curved joint to a large plate which probably 

 represents the protopodite of the limb ; attached to the protopodite is a delicate plate 

 which possibly represents the exopodite (cf. Acanthomunna jJroteus, p. 50). 



The penial filament and the joint by which it articulates with the rest of the limb 

 represent the endopodite ; the exopodite is a thick stout plate which lies in the natural 

 position at the side of the central operculum, it terminates posteriorly in two longish 

 spines of a yellow colour, one arising from the ventral the other from the dorsal side of 

 the joint. 



The third pair of appendages consist of a triangular basal joint with which are 

 articulated a long delicate plate terminating in a filament and a thin gill lamella ; I have 

 not succeeded in separating the latter, so I have only figured (fig. 12) the basal joint and 

 the filament, which probably corresponds to the exopodite ; the latter is of very peculiar 

 shape; it is expanded at its proximal part into an oval widish plate fringed along the 

 outer margin with short slender hairs; it terminates in an excessively attenuated filament 

 which is not so long as the penial filament. 



1 The figure has been inadvertently placed upside down. 



