794 PROF. G. C. BOURNE ON THE 



cerebral commissure, the last named also having connections with 

 the tentacular and median centres. Evidently, the cerebral 

 ganglia have undergone a high degree of concentration and 

 integration, but it is to be remar-ked that if the labial lobe as 

 figured for Alcadia were pulled out towards the bottom of the 

 picture, it would form an elongated labial process from which 

 the labio- proboscidean nerves would be given off at intervals, as 

 is the case in TrocJms and Turbo. In other words, the labial 

 lobe, as it seems to me, is represented, not solely by the little 

 projecting boss from which the labial commissure originates, but 

 by all that part of the ganglion that lies below a line drawn from 

 the lower side of the origin of the cerebral commissure to a point 

 just above the origin of the buccal commissure. 



The buccal ganglia, with their commissures and the nerves 

 originating from them, have been very cori'ectly figured by 

 Bouviei". 



The organ which I have mentioned above as connected with a 

 branch of the left opercular nerve is shown in section in fig. 55 

 (PI. XL.). It is deeply embedded in the muscular tissue of the 

 opercular lobe and lies to the left of the anterior end of the 

 lobe close to the origin of the left columellar muscle. It consists 

 of a cartilaginous plate of subtriangular form, the edges of the 

 anterior apex inrolled ventrally and eventually fused so as to foi-m 

 a short conical tube. This cartilage forms, as it were, the cover 

 of a flattened sac (fig. 55, sac.) lined throughout by an epithe- 

 lium which is thin and composed of a single layer of somewhat 

 flattened cells on the side attached to the cartilaginous plate, but 

 thick and composed of columnar cells beai-ing short stiff cilia on 

 the opposite side. Anteriorly this is continued into the tube 

 formed by the inrolled edges of the cartilaginous plate, and here 

 the columnar ciliated cells form a nearly continuous lining to 

 its cavity. Posteriorly, as shown in fig. 55, the ciliated cells 

 forming the floor of the sac (the reader must understand that the 

 figure is reversed, so that the ventral side is uppermost) rest on a 

 thick basement-membrane, from which a broad band of muscular 

 fibres spreads to be attached partly to the muscular wall of the 

 left side of the neck, partly to the bands of muscular fibres passing 

 over the otocysts. Into this muscular band the large branch of 

 the left opercular nerve penetrates. Taking an anterior course 

 this nerve gives off a branch to the operculai- muscles, but its 

 main trunk is directed towards the anterior tubular end of the 

 organ under consideration, and there passes through a small 

 perforation in the cartilaginous wall and is distributed to the 

 ciliated columnar epithelium lining the cavity of the sac, Dorsally 

 the cartilaginous plate is connected with the left columellar 

 muscle by a stout muscular band passing obliquely outwards. 

 Above this band are seen in fig. 55 the sections of two convolu- 

 tions of a coiled glandular tube, which on the one hand commiuii- 

 cates by a very narrow duct with the antei'ior tubular end of the 

 above-mentioned sac, and on the other hand opens to the exterior, 



