REPORT ON THE CRUSTACEA MACRURA. 225 



margin. The oblique ribs in Penimis commence near the margin and divide at a certain 

 distance, and after a space again subdivide and terminate at the margin where a sin<de 

 hair articulates with every rib at its extremity. The intermediate spaces between the 

 longitudinal ribs are occupied by strong muscles, and those formed by the marginal ribs 

 are occupied by vessels that communicate with the hairs which fringe the inner margin 

 of the scaphocerite ; those on the outer side are upon the under surface, on the inner 

 side of the ribs, whence they probably receive their nourishment. 



This plan is best observable in Hemvpenaeus (PI. L.), in which the circulating vessels 

 are seen to traverse the appendage longitudinally in two lines, and to ramify over the 

 surface in numerous capillaries that are traceable to the small direct lines leading to the 

 marginal hairs. The joint on which the scaphocerite stands is the largest of the second 

 pair of antenna?, and its dimensions show r the importance of this appendage in the economy 

 of the animal. Its purpose I take to be chiefly to balance the animal in a vertical 

 position wdien swimming, but it appears also to be useful in folding over and enclosing or 

 hugging objects against itself, and which may account for a condition that is met with 

 in some specimens, the hairs being lost and the margins thickened as if diseased by much 

 friction. In some this is increased to a larger extent than in others, and sometimes is 

 accompanied by a constriction of the margin (fig. c.) that is suggestive of a permanent 

 variation in form. On the inner distal angle in Penasus there is a tubercular ju'ocess, 

 which in other genera, as Benthesicymus, is developed into a hook with a blunt extremity, 

 and its apex is lodged in a depression on the inner surface of the first pair of antennae ; in 

 those species in wdiich the process is sufficiently developed to be unciform, there is a hollow 

 above, in the ridged structure of the under side of the stylocerite, that does not exist in 

 those species where the process is present only as a tubercle. This hookdike process, 

 which may conveniently be named the " aneecerite," 1 enables the animal, by the 

 assistance of the powerful muscles of the second pair of antenna?, to hold the first 

 pair down more firmly than it otherwise could. In Pcneeus, where this structure exists 

 only as a tubercle, the animal does not roll itself up as it does in Benthesicymus, in 

 which the external tissues are soft, and consequently have less protection. 



The three succeeding joints of the peduncle are small, and only important as being 

 the carriers of a long and slender flagellum, that in some genera, as Aristeus, is three 

 or four times as long as the animal, a feature that appears to be common to those forms 

 obtained from very great depths. 



The mandibles in all the Penseidse are large and powerful organs ; the external 

 portion consists of an incisive margin, wdiich is generally smooth or but slightly dentate, 

 and beneath it a brojfd, circular, molar tubercle ; when the incisive margins meet there 

 is a space between them and the molar tubercles that is occupied by a process from a 

 fleshy mass that overlies them anteriorly. The posterior process of this mass fulfils the 



1 xyx.>i, a liook. 

 (ZOOL. CHALL. EXP. — PART LII. — 18S6.) Eff 29 



