41 



repeatedly and in quite an irrefi;ular manner. Anastomoses 

 between the various branches have been figured by Wilson, 

 but we have not seen these. All along the course of the 

 main branches small twigs are sent out, some no longer than 

 their own diameter, and these grow and invade whatever 

 portions of the body they can find room in. The gonad 

 thus occupies no particular part of the body of the animal 

 except that the position of the external opening is con- 

 stant, but it may be conveniently said to be situated 

 posteriorly to the digestive gland, and as ripening proceeds, 

 to invade every part of the body, where only connective 

 or parenchymatous tissue is found (PI. I., fig. 2, tub. ov.). 

 Simultaneously with the encroachment of the genital 

 tubules, this tissue undergoes absorption or degeneration. 

 The keel-like mass, the "abdomen," (PI. I., fig. 1., abd.) 

 situated medially and posteriorly, is thus completely filled 

 up by the organ, and as maturation goes on, the whole 

 space in the mantle lobes (PI. I., fig. 2, Mn. L.; Mn. B.), 

 between the internal and external epidermal surfaces is 

 filled up by the branching tubules of the gonad and 

 except for the merest trace of connective tissue contains 

 nothing else. Long before spawning occurs these tubules 

 have invaded almost every part of the body (PI. I., fig. 2), 

 even the delicate membrane forming the external wall of 

 the pericardial cavity being overspread by them and 

 becoming opaque. With the act of spawning the whole 

 appearance of the animal may be changed, owing to the 

 emptjnng of these tubules of their contents, and the 

 mantle lobes may lose their thick and opaque appearance, 

 and become thin and transparent. This is the case in 

 the INIussels in some of the beds in the district, but more 

 usually the mantle lobes remain thick and opaque. After 

 spawning it is only by microscopic examination that the 

 condition of the animal can be determined. 



