THR WINDOW-PANE OYS'I'KRS. 51 



The multiple eiidogeny which I have described as occurring in 

 these parasites finds a parallel in the life-historv of the common liver- 

 fluke, where a ciliated free-swimming larva actively penetrates into 

 the hver of a gastropod mollusc and there becomes transformed into 

 a so-called sporocyst, which produces offspring, endogenously. All 

 those larvaB in the hver of placuna which carry endogens are there- 

 fore in the condition of sporocysts. 



The general type to which these parasitic larvii? belong is known 

 as the Cvsticercoid, and a close analogy A\ith the phenomena noted 

 in connection uith the proliferation of the Cysticercoids of Placuna 

 seems to be afforded by the proliferating form named Polycercus by 

 Villot (1883).* This form was discovered by Metschnikoff in the 

 earthworm Lumhricus terresiris in 1868, and was described without 

 being named. An account of it, quoted from Leuckart's work on 

 " Die Menschlichen Parasiten," is to be found in a paper by Profes- 

 sors Haswell and Hill,t describing a new species from the earthworm 

 Didymogaster sylvatica, Fletcher, common under stones and dead 

 timber in New South Wales. These authors say that the infested 

 earthworms usually contain immense numbers of cysts of sizes 

 grading up to 1 mm. in diameter, adhering in clusters to the outer 

 surface of the alimentary canal. Each cyst contains usually 8-12, 

 sometimes 30 Cysticercoids, which have arisen by a peculiar method 

 of budding from a primordial larva. This species was subsequently 

 named Polycercuft didymorjastris by Hill (1894). J In its earliest 

 stages it is a solid, spheroidal mass of small-celled tissue. 



Metschnikoff' s larva may be called Polycercus niloticus, since it is 

 now known to be the bladder stage of the tapeworm Tcenia nilotica out 

 of Cursorius europ'jeus.^ "In its mature condition it consists of a 

 thin-skinned bladder, wliich contains a varying number (up to 13) of 

 small Cysticercoids. Although the latter lie quite free in the interior 

 [of the cyst], and possess, like the ordinary Cysticercoids, the distinc- 

 tive caudal bladder, they are of very unusual origin, inasmuch as 

 instead of developing directly from the six-hooked embryos, they 

 arise by proliferation of the wall of the .surrounding bladder. The 

 bladder is thus the brood-capsule of the enclosed Cysticercoids, and 

 corresponds in some respects to the brood-capsule of the Echinococcus, 



* Meinoire STir les cystiqiies des Tenias. Ann. Sci. Nat. Zool. (6), XV^, 

 18S3 (reference taken from Haswell and Hill). 



t W. A. Haswell and J. P. Hill. On Polycercus, a proliferating Cystic 

 Parasite of the earthworms. Proc. Linn. Soc. N.S.W. (2), VIII., 1893, 

 pp. 365-376, two plates. 



X J. p. Hill. Contribution to a further kixowledge of the Cystic 

 Cestodes. Op. cit.. Vol. IX., 1894, pp. 49-84, 3 plates. 



§ See W. B. Benham. Platyhelmia, &c., in Lankester's Treatise on 

 Zoology. Part IV., 1901, pp. 142-143. 



