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



antennal somite, which in the Brachyura and Macrura is generally closely fused with the 

 two succeeding. 



The ophthalmopoda undergo various modifications of form throughout the order, but 

 the most common condition is that of a pair of pyriform or subcylindrical appendages, 

 the peduncles, each of which generally slightly enlarges towards the distal extremity, 

 where it supports a reniform or hemispherical pigmented organ of vision, the ophthalmus; 

 at the base the peduncle abruptly narrows and is supported on a slender pedicle, which 

 varies in length, as may be observed by comparing that in the genus Eretmocaris 

 (PI. CXLV.) with that in Palsemon or Astacus, where the pedicle almost disappears. 

 In Alpheus and its congeners, Aihanas and Cheirothrix(P\. XCVI. fig. 2a), the peduncle 

 also undergoes diminution. 



This pair of appendages is thus shown to be liable to undergo various changes in 

 each of its parts, and these changes have a tendency to be associated more or less 

 exclusively with the several divisions of the order. 



Among the Trichobranchiata the ophthalmopoda are generally short and supported on a 

 pedicle that is only sufficiently long to admit of the free motion of the peduncle, whereas 

 the ophthalmus is generally hemispherical or reniform, the most normal condition being 

 seen in Homarus, Nephrops, Astacus, and Palinurus, and the greatest departure may be 

 found in the young of the last and in the aborted condition seen in Willemcesia and its 

 congeners. 



In the Phyllosoma shown on PL XIIa., whether it be the young of some one of 

 the Palinuridse or of the Scyllaridse, the ophthalmopod, a short period after hatching, 

 is projected on an extremely long pedicle, which is the more remarkable inasmuch as 

 both in the brephalos condition (PI. XIIa. fig. 1) as well as in the adult stage the 

 organ is short and the pedicle reduced to the smallest condition consistent with free 

 movement. 



In Phoberus the ophthalmopoda are reduced to two small slightly movable processes, 

 with a small globular ophthalmus, as they are also in Nephropsis, while in the aberrant 

 genus Thaumastocheles they are absent altogether, or only represented by two small fixed 

 calcified points. 



During the expedition of the " Travailleur " A. Milne-Edwards took a species that 

 he named Richardina spinicincta, in which the ophthalmopod is reduced to a sightless 

 globe, surmounted by three strong teeth, and in a specimen of Palinurus he found that 

 from the middle of the eye a multiarticulate appendage was produced. 1 According to 

 Leydig 2 the eyes of Cambarus pellucidus (Tellkampf) have neither pigment, rods 

 (bacilli), nor cones, and that while they differ in the adult condition from those in 

 the more normal species, they are comparatively larger in the young than in the adult 



. l Comptes rendus, torn. lix. p. 710, 1864. 

 2 Untersnchungen zur Anat. nnd Histologie der Thiere, 1883. 



