ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 229 



vesicular nucleus ; they are connected with a nerve-fibril, and the 

 free extremity carries a long sensory hair, which traverses the canal 

 in the cuticular layer. The bulbs beyond the proboscis have the 

 cuticle very delicate, and the canal reduced in size, while the hairs or 

 setae are set in a bundle. The nerve-fibres have an elastic and very 

 transparent sheath, and an axis-cylinder, which may be distinctly seen 

 to be made up of fibrils. The fibre loses its sheath at the base of the 

 gustatory organs, and the fibrils separate and, penetrating into the 

 corpuscle, come into relation with the central portion of the gustatory 

 cell. 



" Lung^" of Onchidium.* — M. Joyeux-Laffuie describes the cha- 

 racters and discusses the nature of the organ described by Cuvier in 

 this animal as a lung, which lies in the walls of the mantle, and opens 

 to the exterior at its edge behind the anus. It is formed of two lobes, 

 right and left, united posteriorly by a narrow band containing the 

 orifice. In section the organ is seen to consist of an irregularly 

 areolated tissue, the cavities of which are bounded by bands of 

 muscular tissue, but open freely into each other, and thus to the 

 exterior. The walls of each, cell or areola are lined by several layers 

 of yellowish globular cells, containing a secretion which gives the 

 reactions of uric acid ; the innermost layer is a ciliated epithelium. 

 The muscular layers contain the afferent and efferent vessels. The 

 structure is essentially that of the renal tissues of MoUusca, and this 

 interpretation of its nature is borne out by the development. In the 

 embryo the organ appears near the front edge of the mantle, above 

 and to the right of the heart, like the renal organ of other Gasteropoda. 

 It subsequently becomes an unpaired hollow sac communicating with 

 the pericardium and the exterior, and the walls consist of a single 

 layer of typical molluscan renal cells. During the larval stage it is 

 thrown to the rear by the alteration of shape which the mantle under- 

 goes. It is further observed that the embryo has the characters of a 

 non-pulmonate rather than a pulmonate Gasteropod, in its early-cast 

 shell, in the form of the foot, and the great development of the velum. 

 The respiratory function of this organ being abandoned, the true 

 organ of respiration has to be discovered ; the circulation furnishes 

 the clue to this question. Of the three large longitudinal venous 

 sinuses which receive the blood from the body-cavity, a median 

 one lies along the middle line of the foot, and the other two are 

 lateral and run within the edge of the mantle, near its internal 

 surface. They send out numerous vessels towards its exterior, which 

 break up there into a very elaborate network with narrow meshes, 

 especially developed in the dorsal papillae. The blood passes from 

 this reticulum into two longitudinal vessels, which lead into the 

 auricle. The surface of the mantle is that part of the body which is 

 best adapted to carry on respiration, by virtue of its rich vascularity 

 and the numerous papillas which it bears. These papillae each possess 

 an afferent and an efferent vessel connected by superficial capillaries, 



* Comptes Kcnduis, xci. (1880) pp. 997-1000. 



