b rUOCEEDINGS OF THE 5IALAC0L0GICAL SOCIETY. 



ahi/ssornm, a Chiton found at a deptli of from 150 to 200 fathoms off 

 tlic coast of Norway, were bought for the Royal College of Surgeons. 

 After dissecting these specimens for the purposes of the Museum, 

 it seemed desirable to confirm certain details by the examination 

 of microscopic sections made from the remaining fragments. These 

 sections, cut with the object of verifying certain points in the 

 respective shape and size of different regions of the lateral nerve- 

 cords, revealed upon investigation, as so often happens, other points 

 of interest besides those actually expected, throwing in this instance 

 considerable light upon the vexed osphradial question. 



The individual gills of a Chiton are innervated from the lateral 

 nerve-cord by means of a pair of fine nerves, one of which runs down 

 the outer wall of each blood-vessel.' Both nerves, in contradistinction 

 to the lateral cords from which they spring, are entirely devoid of 

 ganglion cells. Although such an arrangement is the general rule, it 

 does not hold in the case of Hanleya ahyssorum., except for the three 

 anterior gills ; for in that Chiton, from the fourth gill to the sixteenth 

 and last, the outer branchial nerve, that is the nerve situated in the 

 wall of the efferent branchial vessel, is ganglionated in varying degrees 

 (Fig. I, iv-xvi). In the fourth and fifth gills the ganglion cells are 

 confined to the proximal portion of the nerve, forming a small oval 

 ganglion, lying without the gill on the floor of the main efferent 

 branchial vessel. From the sixth gill onwards ganglion cells are 

 present upon the nerve, both before and after its passage from the 

 main efferent branchial vessel into the individual vessel of the gill. 

 The maximum number of ganglion cells is reached about the tenth 

 gill, and from that point to the sixteenth gill the size and extent of 

 the ganglionic masses have a slight tendency to diminish. Looking 

 closer at a well-marked example, say the tenth gill, it will be noticed 

 (PL II, Fig. 1, e.n.) that the ganglion cells are not evenly distributed 

 over the entire surface of the nerve, as in the cords of the central 

 nervous system, but are aggregated into patches, thus forming a string 

 of ganglionic enlargements, giving the nerve a beaded appearance. In 

 the first ganglion, the one, that is, lying witliout the gill in the main 

 efferent branchial vessel (I'l. II, Fig. 1, r/y.), the ganglion cells form a 

 cortex surrounding a central bundle of fibres, but in the portion of the 

 nerve situated within the gill they are chiefly confined to the surface 

 directed towards the cavity of the blood-vessel (PL II, Fig. 2, e.oi.). 

 The ganglion cells in question are small, closely congregated together, 

 and provided with a round nucleus, thus agreeing with the cells that 

 are considered to be characteristic of a sensory ganglion.- The side of 

 the nerve directed towards the exterior is fibrous in structure, and is 

 closely applied to the epithelial covering of the blood-vessel. Before 

 leaving the nerves and turning to the epithelium, I wish to draw 

 attention to a slight peculiarity of the lateral nerve-cord itself : it will 



' Au idea of the general auatomy can be oLtMined from PI. II, Fig. 1. 

 - J. Thiele, " Veber Siuuesorgane der ^eitenlinie und das Nerveusystem von 

 Mollusken": Zeitsclir. Wiss. Zool, xlix, 1890, p. 425. 



