292 WILLIAM A. HASWELL. 



organ. The external aperture is capable of being dilated or 

 contracted by specially arranged fibres of the muscular layers 

 of the body wall. At the narrow end of the sac, where the 

 branches are given off, are two large ganglion-cells (figs. 9 

 and 10, g.). 



Nervous System. 



The cerebral ganglion (PI. XXI, fig. 12, and PI. XXII, 

 fig. 1) is a six-sided body situated immediately beneath the 

 longitudinal layer of muscle on the dorsal aspect, just in front 

 of the pharynx. It consists of a clump of non-nucleated granular 

 material, having ganglion-cells symmetrically arranged around 

 it, with lateral, anterior, and posterior commissures of nerve- 

 fibres. Laterally, it gives off in front a pair of nerve-trunks, 

 each of which divides into three branches entering the tentacles 

 — the middle tentacle being supplied by a branch from each 

 side. Lateral branches pass outwards to supply the sides of 

 the anterior region of the body. Posteriorly, the ganglion 

 gives origin to three pairs of nerve-cords, which pass back- 

 wards towards the posterior end of the body. The ganglion 

 bends downwards laterally towards the ventral aspect of the 

 body, but the origin of the branches are still all distinctly 

 dorsal in position. The first of these — the dorsal (PI. XXI, 

 fig. 12) — are the smallest, and are rather more superficial in 

 position than the others. They leave the cerebral ganglion at 

 its posterior and lateral angles, and run along on the dorsal 

 aspect of the body immediately beneath the longitudinal layer 

 of muscle in the pigmented " nervous " layer, the distance 

 between the two cords being less than their distance from the 

 lateral border. Branches are given off from these cords as 

 they pass backwards, both internally and externally at frequent 

 and fairly regular intervals. The internal branches of each 

 cord pass inwards at right angles to the long axis of the body, 

 and unite with the corresponding branch of the opposite side, 

 sometimes after dividing into two. The external branches 

 bifurcate again and again, the system of fine twigs thus pro- 

 duced anastomosing freely, and giving rise to a fine meshwork 



