10 ■ THE VOYAGE OF H.M.S, CHALLENGER. 



antennfB have a stem composed of five separate joints ; tlie terminal filament is usually 

 shorter than that of the anterior antennae. 



Tactile Organs. — The peculiarly modified antennary hairs first described by Leydig/ 

 and believed by him to be sense organs, exist upon the first pair of antennae only in all 

 the species of Serolis that I have examined. These organs are of two kinds, the most 

 conspicuous being cylindrical jointed hairs, several of which are displayed in PL I. 

 fio-. 4, b ; each consists of a basal portion somewhat hour-glass shaped, which is attached 

 to the inner side of the upper extremity of each of the joints which compose the filament 

 of the antenuules ; this is continued into a delicate cylinder with thin walls, which is 

 always divided into two portions by a transverse septixm, and occasionally seems to 

 consist of three distinctly separated portions ; the distal end of each of these cylinders is 

 formed by a kuol)-like thickening of the chitinous wall. Leydig describes and figures 

 nerve fibres and cells in these olfactory hairs, but the Challenger specimens were not 

 sufiicientl)' well preserved to show these structures. 



As a general rule, only one of these olfactory hairs is found upon each joint of the 

 antennulary filament, but in Serolis antarctica, Serolis australiensis, and one or two 

 other species, two such hairs are found upon each joint. 



The " tactile " organs described by Leydig in the Memoirs already quoted I have only 

 seen in Serolis schytliei and Serolis necera ; on the antennules of these two species one 

 tactile hair only is found upon the distal extremity of the terminal point (PI. V. 

 fig. 5, a) ; it is a short slender hair, the upper half being plumose. Similar hairs are also 

 found over the general body-surface and upon the first joint of the ambulatory appendages 

 (PL III. fig. 11) of many species. 



The mandihles are very strong and powerful, and furnished with a long three-jointed 

 palp which is longer than the mandible itself; the second joint of the palj) is longer 

 than the first joint, and the third, which is very small, is semicircular in shape, and 

 its flat inner margin is furnished with a row of stiff hairs, of which the two distal 

 ones are the longest. These hairs are continued for a short way on to the middle joint 

 of the palp, this part of the joint being of a somewhat greater diameter than the posterior 

 portion, which is devoid of hairs. The basal portion of the mandible is broadest 

 proximally, where it articulates with the head, it narrows abruptly into the distal half, 

 which is not more than one half of its diameter ; the latter is bent at an angle to the basal 

 part, terminates in the masticatory edge which is of a dark brown colour, and is either 

 straight or slightly sinuous, or provided with one or two blunt tooth-like projections ; in 

 adult specimens only are the mandibles thus furnished with a comparatively smooth edge, 

 Avhich appears simply to be due to wear and tear ; in young specimens of all the species 



' Fr. Leydig, Ueber Geruchs- und Gehororgane der Krebse und Insecten, Archivf. Atiat. u. Physiol., 1860, pp. 265- 

 314, Tafn. vii.-ix See also the same Ueber Amphipoden und Lsopoden, Zcitschr. f. viss. ZooL, Bd. xx.\.. Supplement, 

 pp. 225-274, Taf. ix.-xii., 1878. 



