Watt. — On Boltenia pachydermatina. 343 



slowly to a yellowish colour. The gonads of the right side 

 lie along the body- wall, rather nearer the ventral than the 

 dorsal surface. They extend from the anterior to the pos- 

 terior end of the body, at the posterior end make a bend dorsal- 

 wards, and when adjacent to the dorsal surface send off a duct 

 running forwards, and opening into the atrial cavity opposite 

 to the duct from the left side. The gonads of the left side lie 

 in the intestinal loop, their duct opening into the atrial cavity 

 immediately behind the anus. The blocks of both sides are 

 not uniform in size : they diminish in size from the anterior 

 to the posterior end. This diminution in size is often some- 

 what abrupt, so that we can distinguish on each side a larger 

 and a smaller series. The gonads of the right side are, as a 

 rule, more developed than those on the left. The total num- 

 ber of blocks on each side is on an average very nearly equal, 

 there being about thirteen or fourteen on each side ; but on 

 the right side there are about eight larger blocks, on the left 

 only about five. 



In Plate XXXIV., fig. 4, where a view is shown of the 

 body opened from the ventral side, with the branchial sac 

 removed, the gonads of both sides are shown. 



Supplementary Notes on Gonads. — In specimens taken in 

 winter the gonads are olive -green in external view. When cut 

 in section, each is seen to consist of the central white portion, 

 with the green part wrapped round it. Later on in the season 

 white specks appear on the green surface of the gonad ; and still 

 later, in the beginning of summer, the gonads come to consist 

 mainly of a clear-coloured tissue, with white specks imbedded, 

 and with a small green portion gathered round the base. The 

 green portion is seen to consist of small round green masses 

 imbedded in gelatinous substance. These seasonal changes in 

 the appearance of the gonad I shall try to account for in the 

 histological part. 



F. Nervous System. 



The brain (nerve-ganglion) is oblong in shape, with one or 

 more constrictions about the centre ; gives off nerve-trunks 

 anteriorly and posteriorly to the neighbourhood of the oral 

 and atrial apertures. It lies about midway between the two 

 apertures. 



Froni the anterior end a pair of nerve-trunks are given off, 

 which run one round each side of the oral siphon, giving off 

 branches supplying the tentacles, &c. From the posterior 

 end a pair also are given off which go to surround the atrial 

 siphon, sending off nerves to the apex of the siphon. These two 

 nerve-trunks I have traced more than half-way round the 

 atrial siphon. The position of the dorsal tubercle, as I have 

 stated before, is to the right of the brain, and slightly nearer 

 the oral aperture. 



