H. D. SRIVASTAVA                                     189

length, 1 cm. to 160 cm., and in the shape of their mature segments. It is
difficult to explain precisely these specific variations which may be due to
some factors influencing the life-history, such as the individual or specific
resistance of the host or its suitability to the development of the parasite, or
it may be due to differences in the state of maturity of the larvae or may cor-
respond to varieties or strains within the species. Specimens of M. lineatus
with segments broader than long may be referred to M. lineatus var caesta,
while others with segments longer than broad to M. lineatus var litterata.
These varietal differences are merely conventional, as intermediate forms are
not uncommon. The other species, M. perlatus, is usually parasitic in birds.
The testes in this species are mostly external to the excretory vessels.

Though a vague suspicion existed since 1874 when Leuckart pointed out
the similarity of scolices of " Cysticerci " from the abdominal cavity of lizards
with Taenia litterata of foxes, it was Henry [1927] who for the first time proved
that Tetrathyridia are the larvae of Mesocestoides sp. Schwartz [1927] fed
Tetrathyridia from the peritoneal cavity and lungs of a mongoose to a dog
and a cat and forty-six to fifty-one days later obtained adult specimens of a
species of the genus Mesocestoides. Five years later Joyeux and Baer ob-
tained mature worms, referable to M. ambiguus, by feeding Tetrathyridia
from a snake, Elaphe scalaris, to a cat. In both the above experiments the
time of development was very long which might be due to unsuitable
material. Witenberg [1934] carried out feeding experiments on dogs and
jackals and found that Tetrathyridia from the serous cavities of cats and
hedge-hogs and liver of lizards gave rise to mature specimens of M. lineatus
in two to three weeks time. He found that Tetrathyridia elongatus (Blumberg,
1882) from cats is identical with the Tetrathyridia found in hedge-hogs, foxes
and lizards.

Tetrathyridia are highly contractile, flat, semi-transparent bodies, broad-
er anteriorly and tapering posteriorly. They are provided with an invagi-
nated scolex bearing four unarmed suckers, and without a rostellum. The
Tetrathyridia found in mammals are much larger than those parasitic in
reptiles. The term—Tetrathyridia—does not imply a genus but only a collec-
tive group of cestode larvae. They occur either free in serous cavities or
encysted in various parts of the host. Usually they occur in the peritoneal
cavity. In birds they are usually found in the lungs. At times both free
and encysted forms are found in the same host. Their number in a single
host may vary from one to over a thousand. In unsuitable final hosts the
larvae either perish in the intestinal tract or migrate to the serous cavities or
other parts. Occasionally in an unsuitable host a few Tetrathyridia develop
into adult forms, while others migrate into serous cavities and the host is
known as a ' facultative ' host.

A casual examination of a non-poisonous snake— Tropidonotus platyceps—
found in the author's bungalow, revealed the presence of numerous encysted
Tetrathyridia in its serous coat and liver. Subsequent examination of a
large number of specimens of this snake, which is very common on these