96
Anatomy of Acanthostomum burminis
of the body. Before opening to the exterior, however, they show a constant
spindle-shaped swelling, which, in the case of the larger cæcum, measure
0. 060 mm. in thickness and in that of the atrophied one only 0. 012 mm.
DISCUSSION
There exists among the digenetic Trematoda a great diversity of intestinal
cæca. In a typical form the cæca are simple, pass posteriorly along the sides
of the body and terminate blindly at varying distances from the posterior
extremity of the body. The cæca may be branched, either only externally
or both externally and internally. In some cases the two cæca may unite
and form a common cæcum. Various instances of the modifications of the
typical condition may be cited, but these will not be elaborated at present.
Recent investigations have shown that cæca, instead of terminating blindly,
as in the typical case, may communicate with the exterior. In this latter
case also there are three different varieties. Leiper [1908] was the first to
detect, in Balfouria monogama, that the intestinal cæca open into the excre-
tory bladder and. thus communicate with the exterior. This observation
was confirmed by Odhner [1910] in the case of the genus Chaunocephalus and
by the same author [Odhner, 1911] in that of Haplocladus. Looss [1912]
described such a communication in Tetrochœtus and Orophocotyle. Other inst-
ances in this category are Distomum calyptrocotyle, Opecœloides, Allocreadium
annendeli, Ptychogonimus, and the members of the family Accacœlidæ, viz.
Odhnerium, Accacladium, Rhynchopharynx, Accacladocœlium and Accacladium.
Cases are also on record wherein two intestinal cæca unite posteriorly and com-
municate with the exterior by means of an anal tube. This condition obtains in
Opecælus, Opegaster and Anisoporus, members of the family Opecœlidæ.
The third variety of cæcal communication with the exterior is found in Diplo-
porus, Diploproctodceum, Bianium, Schistorchis and some species of the genus
Acanthostomum. In these cases each intestinal cæcum communicates separate-
ly with the exterior by means of an anus situated laterally, at some distance
from the posterior end. The specimen under discussion comes within the
latter category. A glance at the various genera and the species mentioned
above will reveal that the communication of the intestinal cæca with the
exterior occurs quite irrespective of the families to which the forms belong.
Thus the genera mentioned above fall in Accacœlidæ, Echinostomatidæ,
Steringophoridæ, Azygidæ, Opecœlidæ, Srigeidæ Acanthostomidæ and Al-
locreadiidæ and these families are not closely related to each other. Evidently,
acquisition of an anus in distantly related forms is a case of convergent
evolution.
As mentioned before, the chief feature of the specimen under discussion
is that the intestinal cæcum of the right side is very much attenuated. There
are a few instances among digenetic Trematoda where only one intestinal
cæcum is present. Thus in Haplocladus, Unicœcum and Monocæcum there
is only one intestinal cæcum. In this connexion mention should also be made
of the. case of the members of the family Bucephalidæ. In Haplocladus only
the right intestinal cæcum is present, while the left one is completely atro-
phied. The species in question provides a remarkable example wherein the