Watt. — On Boltenia pachydermatina. 345 



into more elongated cells, furnished with cilia. Following on 

 these are small flattened cells, closely resembling the branchial 

 epithelium, which run for some little distance down the groove. 

 Then come the cells of the first glanchdar mass, which is much 

 greater in size than the other two glandular masses. These 

 glandular cells are followed by elongated cells with small cilia, 

 after which comes the second glandular mass. This, as before 

 stated, is of considerably less extent than the first glandular 

 mass, and is followed by elongated cells with small cilia. 

 Next comes the third glandular mass ; and, last of all, on the 

 floor of the groove are elongated cells with very long cilia. 

 The glandular cells are of pecuhar form — broad at the base, 

 and tapering towards the apex. 



The walls of the groove contain very numerous blood- 

 spaces. 



Histology of Gonads. 



The gonads are covered by a continuation of the epithelium 

 lining the peribranchial cavity. Beneath this wall, and lying 

 right round the gonad, is a distinct blood-space. The gonads 

 are ovotestes ; the tubules from each testis open into the vas 

 deferens. The latter runs from the anterior to the posterior 

 end of the body in close connection with and anterior to the 

 oviduct, and at the place where the ducts open into the atrial 

 cavity turns anteriorly for a little way, the two terminations 

 thus forming a fork. In specimens taken in the winter, when 

 the surface of the gonad is of a uniform green, it is found the 

 testis occupies only the lower central portion of the gonad, 

 and a transverse section taken above the centre of the gonad 

 shows merely the riper ova aggregated to one side, and the 

 unripe ova filling the remainder of the cavity. 



Later on, at the time when the white specks appear on the 

 green surface, a transverse section of the free surface shows 

 the tubules of the testes scattered here and there, with the 

 unripe ova in between. As we pass downwards we still have 

 the unripe ova among the tubules of the testis, but at some 

 distance dov?n we come upon the ripe ova lying towards the 

 posterior end of the gonad, with the tubules of the testis 

 around them on the three sides. 



Consecutive sections, taken at the stage when the green 

 colour of the gonad is aggregated round the base, show that in 

 the upper part of the gonad the tubules completely fill the 

 cavity, the unripe ova being absent, and that at the proximal 

 end the ripe ova are collected. These in the earlier stages 

 have a peculiar, somewhat hexagonal, shape, and exhibit dis- 

 tinct vitelline membrane and germinal vesicle, with clear 

 membrane and germinal spot. When fully ripe the ova are 

 enclosed each in a capsule of low cubical cells. 



