6 THE VOYAGE OF RM.S. CHALLENGER. 



4. Turho tmnsenna, AVatson. Station 235 ; 565 fathoms. 



This Turbo has no labial palps either : in this, therefore, as in all the other points 

 examined, it agrees with the already known forms of Turbo. The eyes are pigmented. 



It seems, then, that the presence of labial palps is not general among Trochoids 

 from great depths, and that Trochus infundibulum is, as • far as we know, the only one 

 provided with them. 



As I have already said, among the Streptoneural Gastropods ( = Prosobranchs), 

 Ampullaria and Jeffrey sla^ have two buccal appendages of the same kind as the two 

 lal )ial palps of Trochus infundibulum. 



It may be asked what these structures are, and to what they correspond. 



At first sight we might suppose, as certain authors have done, that they correspond 

 to the two anterior tentacles of the four-ten tacled Gastropods, that is to say, of the 

 great majority of Euthyneura (Pulmonates and Opisthobranchs). This is, however, not 

 the case. 



The fact is, that labial palps analogous to those of Trochus infundibulum and of 

 Ampullaria exist also at different degrees of development in certain forms of four- 

 ten tacled Gastropods ; as among the Pulmonates, in Helix, Bidimus,^ Achatina, and 

 especially Glandina;'' among the Opisthobranchs, in Dolabella neajyoUtana (PI. I. fig. 

 G), Phyllaplysia.^ 



The labial palps are therefore not anterior tentacles. Their origin is to be found 

 simply in the development and lengthening of the two lateral extremities of the snout, 

 which are thus developed for the purpose of adding a sensory organ, and especially an 

 organ of exploration, to that part of the body. Proof of this is found in animals allied 

 to Jeffreysia and Rissoia, where the snout exhibits, on each side of the mouth, a little 

 projection representing the first appearance of the appendages so much developed in 

 Ampullaria for instance. 



In the four-tentacled Gastropods, the nerve of the labial palp springs from a 

 common root with the nerve of the anterior tentacle,'^ while in the Gastropods with 

 two tentacles (Prosobranchs and aquatic Pulmonates [Limnaea), in which the labial 

 palps are represented l)y the " velar area " of Ray Lankester^) this nerve .springs from 

 a distinct root.' 



It is in this last way that the labial palps of Trochus infundibulum are innervated. 

 In Trochus, as in all the Rhipidoglossa, a ventral portion of each cerebral ganglion 



' Jeffreys, Biitisli Conchology, vol. iv. pi. i. fig. 3. 



- Ferussac, Histoire Naturelle des MoUusquea Terrestres, pi. cxlvii. fig. 1. 



" I-eidy, Special Anatomy of the Gastropoda of the United States, pi. xiv. fig. 1, 7 (as " external tentacles"). 

 ■• Fischer, Manuel de Conchyliologie, figs. 330, 331, p. 569. 



^ Leidy, loc. cit., pi. xiv. fig. 1, 10 (Olaiu/iiia). Sarasin, Uebev drei Siunesorgane uud die Fussdriise ciniger 

 (liistropoden, Arb. Zool. Inst. Wiirzhurg, 1883, pi. ix. fig. 10. 



<"' MoJIusca, Encyclop. Brit., 9th edition, vol. xvi. fig. 70, p. 660. 

 ' Bouvier, he. cit., pi. v. fig. 19. 



