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



The situation of these organs at the base of the inhalent siphon, and near the gill, 

 makes it probable that their function is to test the quality of the water which comes 

 through the siphon to bathe the gills. They may therefore be compared to the osphra- 

 dium (the olfactory organ of Spengel) of Gastropods. The ganglion at the base of the 

 tuft in Tellina balthica, and of the tentacle in Yoldia and MaUetia, really corresponds 

 exactly to the " siphonal " ganglion observed in many other siphonate Pelecypoda 

 {Cythevea,^ Solen^ Mya,^ Lutraria*), and more or less separated from the visceral 

 ganglion. 



Tliis " siphonal " ganglion is the osphradial ganglion of Gastropoda and of 

 Nautilus. In the Pelecypoda without siphons (^?-ca, &c.), the sensory epithelium is 

 situated directly above the visceral ganglia." This has led Ray Lankester wrongly 

 to conclude that the latter are the osphradial ganglia {" olfactory"), and that the supra- 

 (Bsophageals are cerebro-pleuro-visceral ganglia.^ But in the siphonate Pelecypoda the 

 sensory epithelium is situated above the siphonal ganglion, and in certain forms 

 (MaUetia, Tellina) this special organ is complicated by the presence of a large tentacle 

 or a tuft of small ones. 



The foot is similar to that of the allied forms already known, — Yoldia, Leda, 

 Nucula, — and presents a ventral plantar surface greatly resembling the creeping foot of 

 Gastropods. The posterior retractor muscle of the foot extends from the posterior 

 adductor muscle to the visceral mass. 



The mouth has two lips, which are continued in well-developed labial palps {&) 

 stretching behind the foot. The situation and aspect of these palps, in the whole group 

 of Nuculidse, have led to their being taken for gills, as, for instance, by Sars in his 

 Yoldia ohtusa,'' and Thiele in Nuctda,^ where he has mistaken anterior for posterior. 

 The two palps on each side possess a common posterior appendage (c), very long, but 

 contracted and rolled up in the figure. 



The gills of MaUetia, and of the whole group of Nuculidse, have a structure different 

 from that of typical Pelecypoda, as Mitsukuri first pointed out in Nucida and Yoldia.^ 



In MaUetia the structure of the gill is even simpler than in Yoldia, the lamellse 

 being much less numerous and less compressed. 



^ Duvernoy, Memoires sur le systfeme nerveux des moUusques acephales, Mem. Acad. Sci. Parii, t. xxiv. (1853), 

 pi. xi. xii. fig. 3, g'. 



^ Blanchard, Observations sur le systems nerveux des inoUusques acephales testaces ou Lamellibranches, Ann. Sci. 

 Aal. (Zool.), ser. 3, t. iii., pi. xii. fig. 1. 



■'' Duvernoy, loc. cit., pi. xi. xii. fig. b, g'. 



* Ibid. pi. xi. xii. fig. 6, g' . 



' Spengel, Die Geruchsorgane und das Nervensystem der Mollusken, Zeitschr. f. u-iss. Zool, Bd. xxxv. p. 374, 

 pi. xvii. fig. 13. 



" Mollusca, Eneycl. Brit., 9th ed. vol. xvi. p. 693. 



' On some Remarkable Forms of Animal Life, i., pi. iii. fig. 20, e. 



" Die Mundlappen der Lamellibranchiaten, Zcitschr.f. wiss. ZooL, Bd. xii., pi. xvii. fig. 17, k. 



' On the Structure and Significance of some aberrant forms of Lamellibranchiatc gills. Quart. Joiirn. Micr. Sci., 

 vol. xxi., 1881, p. 595. 



