JOU.SSEAU.MIA. 25? 



spermatozoa and developing ova were present in the gonads, and in about half a dozen 

 cases the gonads contained ova only and were enormously enlarged, displacing the 

 other viscera in the visceral mass. It would appear that Jousseaumia is also, to a 

 certain extent, paedogenetic, for I discovered ripe spermatozoa in a considerable number 

 of young forms which were clearly immature as regards the structure and development 

 of the shell and gills. 



The gonad itself shows very few traces of paired structure, and varies very much 

 in shape and extent, according as it contains spermatocytes and spermatozoa only, or 

 developing ova or ripe ova. It may be described as consisting of a median vestibule 

 lying in front and ventrad of the kidney and pericardium, opening behind by paired 

 ducts on the reno-genital papilla?, and produced anteriorly into a dorsal and a ventral 

 tubular diverticulum. The diverticula are lined by the germinal epithelium, from 

 which first spermatocytes, then, when the spermatozoa have ripened, oocytes, are 

 produced. In the first stage, when the protandric phase is in evidence, the extent of 

 the gonad is small, as is shown in fig. 1. The dorsal diverticulum extends forward 

 and upward from the vestibule below the floor of the anterior part of the pericardium, 

 and is continued forward below the rectum as far as the point where the latter bends 

 sharply back on itself. The ventral diverticulum is a very short tubular outgrowth 

 from the lower and anterior face of the vestibule, and lies ventrad of the stomachal 

 caecum. The dorsal or anterior end of the dorsal diverticulum is bifurcated, and the 

 two branches often lie on either side of the rectum, this and the existence of paired 

 gonaducts being the only evidences of paired structure in the body of the gonad. The 

 ventral diverticulum is never bifurcated. Spermatogenesis is effected mainly, though 

 not exclusively, in the dorsal diverticulum, which in many specimens is filled with 

 spermatocytes and spermatids in different stages of development, but the state of my 

 preparations did not admit of my making minute investigations on this subject. The 

 vestibule, in this phase, is filled with ripe spermatozoa, and at a somewhat later period 

 the whole gonad contains a mass of ripe or nearly ripe spermatozoa. This was the 

 most common condition in the numerous specimens I examined ; only in two of them 

 could I find evidence of the simultaneous formation of ova and spermatozoa, and in 

 those there were many ripe and a few developing spermatozoa in the dorsal 

 diverticulum, the vestibule was filled with ripe spermatozoa, and developing ova were 

 observed in the ventral diverticulum. The more usual course appears to be that the 

 protandric phase is followed by a short resting stage, during which the diverticula of 

 the gonad are empty and reduced in size, though the vestibule may remain full of 

 spermatozoa. This is succeeded by an active development of ova in both diverticula, 

 which become enormously distended and push their way forward among the viscera, 

 displacing the latter to a very considerable extent. Posteriorly the vestibule 

 bifurcates to form the right and left gonaducts. These ducts, wide at first (fig. 24, 

 go.d.), rapidly diminish in diameter, and passing outwards and backwards, open just 

 to the outside of and behind the renal apertures on the reno-genital papillae on either 



2 L 



