STRUCTURE OF NEPHRIDIA OF THE MEDICINAL LEECH. 287 
of its segments. They have, therefore, though with but little 
reason, been called " segmental organs,’ a name which is 
just as suitable for many other organs. Each of them con- 
sists of a closely-coiled or loop-like canal (Schleifen canale) , 
which has an internal opening, often peculiar in form and 
always ciliated, and which opens at the other end on to the 
surface of the body. This canal is sometimes similar in 
character throughout its whole length, or but slightly dif- 
ferentiated ; frequently several segments may be made out 
in it, which generally correspond with those already de- 
scribed in the Platyhelminthes and Rotatoria. The inner- 
most portion, on which the opening into the coelom is placed, 
is ordinarily the longest ; it is distinguished by its funnel- 
like or rosette-shaped orifice. In the next portion the walls 
may be seen to be glandular in structure. The last portion, 
which is frequently widened, is provided with a layer of 
muscle ; it almost always opens on to the ventral surface. 
These organs are no more purely excretory in function than 
they are in other Vermes, for we not unfrequently find them 
entrusted with other functions. 
'^Tn the Hirudinea these organs are preceded, in the em- 
bryonic stage, by three pairs of looped canals, which are not 
connected with those formed at a later stage ; they are found 
in the posterior half of the ventral surface. In structure 
they are similar to, but simpler than, the permanent canals, 
and disappear after these are developed. This most impor- 
tant fact shows that the looped canals of the Annulata 
cannot be regarded as the direct hornologues of the excretory 
organs of the lower Vermes; at the same time arises the 
question. Are the looped canals of those Annulata, which 
show no signs of primitive organs of this kind, comparable 
to the permanent looped canals of the Hirudinea, or only to 
the primitive ones ? 
In their more special characters there is great variety 
among the Hirudinea, the canals in one division having no 
internal opening. Instead of this they begin with a closed 
portion in the form of a loop, which consists of numerous 
canals united with one another into a labyrinth {Hirud'o). 
From these looped organs a single canal is given off which 
opens by a vesicular enlargement on the surface of the body. 
In others {Clepsme^ Nephelis) the labyrinthine portion is 
present, but it has an internal opening, which projects into 
the lateral blood-sinuses of the body.” 
I may be allowed further to cite a statement of Mr. 
Balfour’s relative to the nephridium of the Leech, which will 
serve to illustrate the view generally entertained as to its 
