ZOOLO&Y AND BOTANY, MIOBOSCOPY, ETC. 855 



The gland is not compact and confined to a definite area, but it extends 

 over nearly the whole of the part which may be strictly called the 

 trunk. Separated from the integument by a delicate layer of con- 

 nective tissue, it forms a system of ducts which anastomose with one 

 another, and have their inner wall developed into csecal processes 

 which are set vertically to the surface of the body, and extend into 

 the connective tissue. 



The genital products are developed on the walls of the caeca, and 

 spermatozoa and ova are developed side by side in the same caecum. 

 They appear to arise from epithelial cells which may be considered as 

 the sister-cells of those which invest the walls of the genital ducts. 

 Their prime origin would appear to be ectodermal, and it is a very 

 difficult matter to make out any mesenchymatous portion. As in 

 other cases, a cell is entirely metamorphosed into an egg-cell, while 

 the spermatozoa arising from one mother-cell form a characteristic 

 aggregation. The ducts of the gland communicate either directly or 

 indirectly with a chief efferent duct, which opens at the anterior 

 termination of a cleft which extends along the muscle of the valves. 



The organ of Bojanus so far agrees with the genital organ that it 

 is not, as in other Lamellibranchs, compact, but is formed of an assem- 

 blage of ducts and caeca, which form a flattened layer of great extent, 

 but of slight thickness. The wall of the cavity into which the pro- 

 ducts of these two glands open also has an excretory function, and it 

 may therefore be spoken of as the urinary chamber, and its ducts as 

 the ureter. There is no communication between the orifices of the 

 genital and renal organs. The " reno-pericardiac canal " effects a 

 communication between the urinary chamber and the pericardium, and 

 it seems probable that the auricles have some excretory function. 



At the time when the ova are laid they are not only fecundated 

 but have passed through the earlier stages of segmentation, but the 

 sperm necessary for this fecundation does not arise from the same 

 oyster. The water which passes over these Molluscs brings from 

 oysters the escaped spermatozoa ; some of these pass into the cavity 

 of the mantle, penetrate the generative orifice, and not only make 

 their way into the principal duct, but also into the larger branches 

 connected with it. The oysters of the Eastern Scheldt may have fry 

 in their gills when they are only two years old, but, as a rule, the 

 oysters with fry are older than this, and those of four or five years 

 have the most. Similarly, oysters two years old may produce sperma- 

 tozoa, but as a rule, these last arise from older forms. As has already 

 been discovered, oysters of one year may develope spermatozoa, and 

 we find in the Eastern Scheldt that the number of oysters which 

 develope spermatozoa is larger than that of those which give rise to 

 ova. The eggs of a mature oyster are, if properly developed, all laid 

 at once ; the spermatozoa seem to be evacuated during a longer period. 

 The evacuation of ova appears to exceed that of spermatozoa. Culti- 

 vation does not show itself favourable to the procreative capabilities 

 of the oyster. In old examples the liver is much more developed than 

 in younger specimens, and this in correlation with the degeneration 

 of the reproductive organs. 



2 A 2 



