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



"false gill" — pennate, but attached to the wall almost throughout its entire length and 

 breadth, — the posterior "true gill" with leaflets along one side. These thin leaflets, 

 which form the functional gill, lie closely together, arranged along a curved transverse 

 line, and slanting down from the base of attachment into the branchial cavity. The 

 leaflets vary in breadth ; in the more typical genera (Marsenia, Marseniella, Mar- 

 seniopsis) they bear laterally either transverse ridges or small secondary leaflets. Behind 

 the gill, on the roof of the cavity, lies a kidney of some considerable length {Marseniella), 

 which may, however, be smaller, and situated on the left. To the left, at the end of the 

 kidney, the pericardium is seen. Behind the gill there is usually a space covered by the 

 whitish " secreted substance " of the kidney, or of the " foliated gland," or of both. Lastly, 

 behind gill and kidney, over the whole breadth of the branchial cavity, there lies the 

 usually broad foliated gland. On the roof of the cavity, in a more median and posterior 

 position, or else to the left, lies the external, branchial aperture of the kidney and foliated 

 gland. At the right-hand corner of the branchial space is situated the more or less pro- 

 truding anal papilla; behind this in the female dioecious Marseniadse lies the vulva, 

 and in the monoecious forms (Marsenina, Onchidiopsis) the female generative aperture. 



The lower part of the body, below the branchial cavity, is smaller than the superior 

 portion, and includes the central nervous system, the whole anterior portion of the diges- 

 tive organs, and a large part of the vas deferens. The head is somewhat large and 

 flattened, truncated in front, sometimes slightly indented, and drawn out on each side into 

 an anteriorly-directed tentacle. The tentacles, springing from the side of the head, are some- 

 what flattened at their base, but are otherwise conical or cylindrical. The eyes are nearly 

 sessile at the base of the tentacles, or seated on short ophthalmophores. The external 

 mouth-opening is situated on the under surface of the head, usually at a little distance 

 from the anterior end, and forms a transverse slit ; sometimes {Marsenina, Onchidiopsis) 

 it lies, further forward, and is then a more roundish opening. Through the external mouth 

 the rostrum is protruded. The foot is powerful, 1 and with the exception of the end of the 

 tail, is wholly overlapped by the border of the mantle, on which, however, the animal does 

 not seem to rest when creeping. 2 The foot is somewhat long and narrowed posteriorly, 

 ending in a rounded point. The anterior border exhibits a deep groove, in which the 

 pore of the foot-gland opens medially. The edges of the foot project but slightly, 

 and the tail is not short. The border of the foot is sometimes peculiarly thickened 

 {Onchidiopsis). The penis is always strongly developed, and though not retractile, can 

 be hidden, during quiescence, within the limits of the branchial cavity. The whole organ 

 is more or less compressed, usually plano-convex, and with the edge bent or somewhat 



1 Quite erroneous is the statement made by Adams, L. Reeve, and especially by Keferstein (Bronn, El. u. 

 Ordn. d. Thierreichs, Bd. iii., Malacozoa, 1864, p. 1057), according to which the foot "almost entirely conceals 

 the shell," and the enveloping mantle is regarded as a development of the Lobus operculigerus. 



2 This seems, however, to be the case in Pyrula; cf. Orsted, Yid. Meddel. nat. Foren. i Kjtjilerihavn, 1850, 

 p. 9. 



