REPORT ON THE MOLLUSCA. 5 



It may be said, therefore, that the rudimentary character of this " eye " consists — 

 1st, In the disappearance of the refractive media. 

 2nd, In the disappearance of pigment. 

 3rd, In the change of structure in the nervous elements. 



Trochid^. 



2. Trochus inftindibulum, Watson. Station 146 ; 1375 fathoms. 



This Trochus, drawn out from its shell (PI. I. fig. 3), at once exhibits a character- 

 istic which distinguishes it from all other animals of the same genus and of the same 

 group ; the head bears on each side of the mouth («) a large labial palp (6), analogous 

 to the tentaculiform appendages of Arnpullaria and Jeffreysia. 



Otherwise the head is formed as in other species of Trochus. The tentacles (c) are 

 strong and still long, though a little contracted. 



The eye is by no means rudimentary. It is situated at the base of the tentacle, 

 on the outer side, and is borne on a short stalk. Neither in size nor form is it distin- 

 guished from the corresponding organ in the other allied species. Finally, it is much 

 pigmented. It is thus situated and formed like that of the littoral Trochus. 



The foot {g) and the epipodium also exactly resemble the corresponding parts in 

 the littoral species. 



As in the latter, the epipodium includes an anterior portion formed by a cervical 

 lobe (A) and a less prominent posterior epipodial line extending to the operculum, and 

 l)earing five tentacles (i). These last are large and rather long. This epipodium is 

 therefore similar to that seen in all the Trochoids, and, as in these, its pedal nature 

 cannot be doulrted. 



The pallial cavity (c) scarcely differs from that of the littoral species of Troclms. 

 The gill is well developed, and is exactly similar to that of species found in lesser 

 depths. So also with the hypobranchial gland, the osphradium, and the kidney. The 

 anus {f) opens a little farther forward than is usual among known species. 



It would be interesting to find out whether other species of the same genus, or of 

 the same group, obtained from great depths, possess the same lal)ial palps as Trochus 

 infundibulum. With this end in view I have examined (1) Trochus rhina, (2) Turbo 

 transenna. 



3. Trochus rhina, Watson. Station 75 ; 450 fathoms. 



This species possesses no appendages corresponding to the labial palps of TrocMis 

 infundibulum. Its structure is analogous to that of littoral species. The eyes are 

 pigmented and well developed ; the gill is normal. 



