ADCLT HEAD KIDNEY OF BDELLOSTOMA STOUTI 851 
istic, however, is wanting, for at no time is there a glomus in the 
coelon opposite the nephrostomes. 
Later the tubules back of segments 30-33 (this point also vary- 
ing) lose their connection with the body cavity ; or to be more 
accurate, the small coelomic pocket into which each of taese tu- 
bules open become cut off from the general body cavity, and forms 
the distal end of the tubule. In the end of each tubule thus 
formed, that is, in a cut off portion of the coelom, a glomerulus 
appears. This portion of the organ has now the structure of a 
simple mesonephros, a structure which it retains throughout 
life. However, a few of the more posterior tubules degenerate, 
and, as will be mentioned below, two or three of the a iterior ones 
become incorporated into the head kidney of the adult. 
The tubules in front of the ones just described retain their con- 
nection with the coelom, and for a time they also retain their 
segmental arrangement. But later the gill slits, which are at 
first far forward, shift their position backwards, and in so doing 
they crowd before them the anterior end of the kidneys, so that 
all of the open tubules (17 or 18 in number) and 2 or 3 of those 
which have been cut off from the coelom and in which glomeruli 
have been formed, are crowded together into a compact body 
occupying the space of one or two segments. It is this body which, 
after undergoing some f urther changes, forms the head kidney of 
the adult. It will be seen that this is in a sense a composite struc- 
ture, since it is formed of two kinds of tubules. The first develop 
into the main body of the gland, while from the second is formed 
what may be called the glomerulus of the head kidney. This is 
the only structure of the kind found in the organ, as neither glomus 
nor glomeruli are formed in connection with the open tubules. 
In the present work adults ranging in length from 22.8 cm. 
to 56.5 cm. were used, and also two young individuals, the one 
7.5 and the other 7.9 cm. in length, in which the intestine still con- 
tained an abundance of yolk. While the head kidney in these 
small specimens has the essential characteristics of the adult they 
are still in some ways less complicated, and formed a sort of 
transition between the oldest embryos before studied (Price, 
'04) and the adult. 
