REPORT ON THE CRUSTACEA MACRURA. 223 



is reduced to a rudimentary condition, or is visible only in the brephalos stage, except 

 in those forms, such as the terrestrial Isopoda, where the entire organ is deteriorated in 

 character. The first joint of the peduncle is the most important of the three, since it 

 contains the acoustic organ, and is also modified in different genera to support and 

 protect the eye. 



In Pcna>us this organ is more perfect in its several parts than in any other genus 

 of the family group, where the tendency is generally to diminish the value of its parts. 

 In its more perfect condition it is about one-third as broad as it is long, the form 

 being something like a flattened cylinder ; this shape is caused chiefly by the depression 

 for the lodgment of the ophthalmopod, and the surface is thinned out accordingly ; in 

 some genera, as Aristeus, and some species of Gennadas, the depression is great; in 

 others, as in Sergestes and its allies, it is almost wanting. In these genera the first joint 

 is short, and the second and third are proportionately increased in length. In Pendens 

 this joint is stout at the base where it has a free articulation with the metope ; the 

 thickness is continued on the inner side to the extremity, and the margin is straight 

 and lies in contact with that of its fellow. Near the base of the joint on the same 

 side, within the margin, there arises an appendage that is first laterally, and at its 

 distal extremity vertically, compressed ; it has the margins, especially towards the 

 distal extremity, copiously fringed with hairs. In some genera this appendage, which 

 for distinction I call the prosartema, varies in length and form, and sometimes is 

 rudimentary or absent. In Sicyonia it is short, rigid, and sharply pointed. In Aristeus 

 it is represented by a small tubercle supporting a tuft of hairs ; whfle it is absent in 

 Benthesieymus, Gennadas, and Sergestes. 



The upper surface of the same joint, as it exists in Penmus, is of extreme tenuity, 

 but the inner and outer margins are strengthened by a longitudinal rib that reaches to 

 the distal extremity, where on the outer side it terminates in a sharp tooth or stylocerite 

 of more or less importance, and is frequently fringed on the upper and lower sides with 

 long hairs. This styliform tooth varies in form and length in different species and 

 genera, being sometimes short and thick, as in Benthesieymus, but always terminating in 

 a sharp point, even in Sergestes, where it is the least developed. In others it extends 

 beyond the distal extremity of the joint, and frecpiently lies so closely in contact with 

 the margin as at first sight to appear to be in connection with it. Between the two 

 margins is a depression into which the ophthalmopod falls when at rest, a prosartema on 

 the inner side overlying and protecting it, and a copious fringe of hairs surrounding it on 

 all sides, their direction being always towards the eye. The hairs, which at the base are 

 very numerous and closely packed, form a thick mat, protecting both that portion of 

 the eye and also a small orifice that opens into the chamber containing the auditory 

 apparatus, which is enclosed within a calcified fold of the external tissue. The second 

 and third joints are generally cylindrical in form, but in Penseus they are rounded on 



