FOOD VAIyUEJ OF SEA MUSSEES. 
89 
The kidneys, or so-called organ of Bojanus, consist of two symmetrical sacs on the 
ventral side of the body situated one on either side of the foot. Each extends backward 
to its opening, which is located on the inner side of the point of attachment of the gill 
just anterior to the posterior adductor muscle. 
The circulatory system is well developed and completely closed as in all other 
mollusks. The heart lies in the mid-dorsal region in a pericardial chamber. From the 
heart a single large blood vessel is given off, which passes forward as the anterior aorta. 
It breaks up into a network of arteries that ramify all through the body. The blood is 
collected into a large, longitudinal vein on the ventral side of the body, from whence it 
passes through the kidneys to the gills and finally to the heart. The blood is colorless. 
The nervous system, as in other lamellibranchs, is made up of three pairs of gang- 
lionic centers connected one with the other and giving off nerves to supply the various 
surrounding organs. One pair is located in the head region with a ganglion on each 
side of the gullet, another in the foot, while the third, just ventral to the anterior edge 
of the posterior adductor muscle, supplies the digestive and reproductive organs, heart, 
gills, and posterior portion of the mantle. 
The reproductive system is much more extensive than is found in most other mol- 
lusks. It is made up of a complicated branching network of canals which radiate 
throughout nearly the entire body. Internally each canal ends in a pocket or fol- 
licle. Externally the canals open out on either side of the body through a genital 
papilla which is at the inner point of attachment of the gills in front of the posterior 
adductor muscle and just in front of the kidney opening (pi. xix, fig. 4). Since there is 
no definite organ which can be designated as an ovary or testis, it is impossible during 
the quiescent period to determine the sex of an individual. In mussels from Woods 
Hole, Mass., genital products were found developing in these canals during the early 
spring and summer months. (Compare fig. 1-4, pi. xxm). According to Williamson 
(1907) the eggs arise from certain minute, brown-colored cells which he found present 
in the mantle of the female. My own observations are to the effect that the sexual 
products are formed by a process of budding from the cells lining the walls of the genital 
canals. At first the cells formed are extremely small and undergo rapid division. 
After a time division stops and the cells enter upon a period of growth. By the time 
the sperms and eggs are ripe they occupy almost the entire portion of the mantles, which 
are greatly distended by them. They fill the floor of the pericardial region, the wedge- 
shaped abdomen and cover to greater or less degree the outer walls of the digestive gland 
(pi. xix, fig. 3). 
REPRODUCTION. 
With such an extensive genital system the mussel is capable of producing an 
enormous number of germ cells. For the past two summers between 200 and 300 
mussels were kept in a shallow trough of running sea water where the process of egg 
laying and fertilization could be readily observed. The extrusion of the sexual elements 
on the part of two or three individuals began within an hour after bringing them in 
from the natural beds, and as time passed the number of spawning individuals 
