Crustacea. 



6677 



them. The organs adapted for these senses are those which are 

 developed as antennae. Of these, in Crustacea, there are two pairs; 

 the one internal, or anterior; the other external, or posterior. These 

 organs differ both in form and relative length in different families and 

 genera ; but throughout the class they are adapted, the anterior 

 to hearing, the posterior to smelling. 



The anterior pair are always erect and on the watch, — jerking, 

 moving and in play. They are universally formed of a peduncle 

 supporting one, two or more filaments. In the lower forms these 

 filaments are generally long and thread-like ; in the highest they are 

 very short, but the shortness consists in the decreased size of each 

 segment, rather than in the number of the segments. Through the 

 many forms of antennae there appears to be one persistent scheme, 

 however varied the aspect of the individual structure may be. The 

 peduncle consists uniformly of three joints or segments. The first, 

 that is, the one nearest the head, carries the acoustic apparatus, 

 which consists, in the crabs, &c, of a bony cell that is circular, with 

 depressed sides, and attached at one point only to the internal sur- 

 face of the walls of the antennae. The apparatus is supplied with 

 nerves direct from the supra-cesophagal ganglion. In the lobster 

 species the apparatus is long instead of round, and not so large in its 

 relation to the organ ; and Dr. Farre says it is always filled with 

 sand, which, he presumes, acts the part of otolithes. The sand 

 1 believe not to be constant, and where presentit is the result of 

 unintentional deposit ; but it affords evidence that there is an orifice, 

 small, though connecting the internal structure with external con- 

 dition. Professor Huxley has discovered an apparatus of a similar 

 character to that which Dr. Farre found in the lobster; but I have 

 failed to perceive anything of the kind in the antennae of the 

 amphipod and isopod species. This probably arises from the less 

 perfect development of the part both in size and structure, rather 

 than from an absence of such apparatus. Again, upon the anterior 

 antennae — so that the absence is a circumstance to be commented 

 upon — there exists, upon one of the filaments, and one only, however 

 numerous they may be, a series of organisms that look like trans- 

 parent hairs, but differ in construction from the true hairs with which 

 they are mixed. The)' are not more slight, but are formed of a tissue 

 that is evidently membranous and very thin ; they are often divided, 

 sometimes branched, and assume many different forms often charac- 

 teristic of genera or species. The Crustacea on which they have not 

 been perceived are few, the most conspicuous of which are those of 



