ANATOMY OF TANIA MEDIOCANELLATA. 21 
and fibrous diaphragm, bearing out in these divergencies the 
normal development of the segments from the head down- 
wards on the point of the body structures preceding the pro- 
duction and development of the viscera, and evidently also 
defining them. 
General remarks.—The main function of the zooid is that 
of reproduction, all else being but subservient to it. The 
points of contact between the zooid and the trematode are so 
marked as to link the one with the other in the animal 
series, to cause the cestoid colony to be regarded as a com- 
pound fluke-worm. I have said nothing in the preceding 
pages on the existence of a nervous system, for the reason 
that clear anatomical details were not forthcoming to warrant 
the statement of its presence; yet, on the other hand, there 
was faint evidence of the existence of a ganglion in the head 
in the form of a circular nucleated cell mass in the centre be- 
neath the suckers. But whether the presence of nerve-centres 
can or cannot be demonstrated there can be no question, 
from subjective evidence, of structures set apart for the 
co-ordination of certain movements and performance of certain 
functions; for example, a centre must be present towards the 
head for combining the sucker action, and also one in each 
segment for regulating the movements necessary in copu- 
lation, also connecting fibres must pass from one segment to 
another for mutual action to this end, for by analogy we 
must regard the zooids, although hermaphrodite, as not self- 
impregnating, and hence consensual action is required. 
The genital pit of one segment could be brought into 
contact with that of another a short distance removed by the 
curving and bending up of the colony to one side, and by 
this action a transposition of their respective orifices would 
result, the penis of the upper segment would correspond to 
the vagina of the lower, and vice versd, and mutual impreg- 
nation would ensue. 
It is also noteworthy that the diameter of the vagina gene- 
rally is zj55 In., while the smallest size of the mature egg is 
=i, 1n., ora relation of the latter to the former of rather more 
than 3 to 1; the egg is unyielding, and the delicate structure 
of the tube neither suggests elasticity nor contractility ; in all 
the numerous sections I have made I have never found an 
egg within the canal, while the uterus is distended with them 
close to the point of communication; and hence I infer that 
the function of the vagina does not include the discharge of 
the ova, but is limited to the reception of the male organ, for 
which the muscularity near the external orifice succeeded by 
the dilatation is well adapted, and transference of the seminal 
