852 
GEORGE C. PRICE 
More than two dozen head kidneys, in some cases ioclading also 
the anterior end of the functional kidney, were sectioned and 
mounted in complete series. While these agree in essential fea- 
tures there is still much variation ia detail, so much that an exact 
descriptioQ of one would not answer in detail for any other. Liv- 
ing as well as preserved material was studied. 
In the adult, as is well known, the head kidne3^s are situated 
a little to either side of the dorsal aorta, and project into the dor- 
sal part of the pericardial cavity. They are intimately connected 
with veins returning blood from the anterior part of the body, the 
left with the anterior cardinal, or rather with a wide diverticulum 
given off from this vein just before it reaches the heart; the right 
with a similar diverticulum given off from a vein which empties into 
the portal heart. This vein is called by Weldon and by some other 
authors the anterior portal, but from its position and distribution 
it looks to be the fellow of the cardinal of the opposite side, al- 
though it does not extend nearly so far forward. The relation of 
the head kidney to the vein is well shown in fig. 2. In Bdellos- 
toma fosteri, according to Weldon, the head kidneys are likewise 
connected with veins returning blood from the anterior part of 
the body, while in Mj^xine they are connected with the posterior 
cardinals returning blood from the posterior part of che body. 
The size of the head kidney varies with age, although it 
is relatively small even in large animals. In one specimen 24.7cm. 
long the head kidney measured 3 mm. long by 1 mm. broad, 
while in one 45.6 cm, long it measured 8mm. b} 2 mm. The ratio, 
however, between the length of the individual and the size of the 
head kidney is not constant; nor are the two organs of the same 
size in the same individual, sometimes theright being larger and 
sometimes the left. 
As a rule tne organ forms a single compact body, although this 
is not invariably the case. In one instance, for example, a small 
bunch of tubules at the anterior end was separated from the main 
body by an interval of more than a millimeter. Development offers 
a ready explanation for cases of this kind. In some of the older 
embryos one or two of the anterior tubules were observed to be 
entirely separated from the rest and from the duct, and it seems 
