1920.] R. B. Seymour Sewell : Mesocoelium sociale. 93 



(2) Stylet glands, six or more on each side, between the 



acetabulum and the phar3aix, 



(3) Excretory bladder bicornuate. 



(4) Very short prepharynx and small pharynx. Oesophagus 



(when developed) short to, of medium length. Intestinal 

 caeca (wdien present) reaching to posterior end of the 

 body. 



Subsequentl>- (1919 (a), p. 275) he described the excretory 

 system in detail in Cercaria polyadena and showed that it possesses 

 the 2x6x3 formula. 



All the arguments that Cort adduces in support of his view 

 regarding the development of Margeana califormensis apply with 

 equal force to Mesocoelium sociale Liihe : here likewise the relative 

 size of the two suckers, the characters of the digestive tract, the 

 type of excretory bladder and the 2 X 6 X 3 type of excretory tubules 

 and flame cells render it probable that this species also possesses a 

 Polyadenous cercaria as its larval stage ; and moreover the presence 

 in the adult worm of true salivary glands, as I have shown above 

 (p. 86), comparable in every way with the stylet-gland of the 

 cercaria, serves to still further strengthen the belief that in the 

 case of this species also the cercaria will be found to belong to 

 the Polyadenous sub-group of the Xiphidiocercariae. 



Classification. 



The genus Mesocoelium was created by Odhner (1911, p. 88) 

 in order to accommodate this species^ which was described by lyiihe 

 (1901, pp. 171 — 173, fig. 5) under the name Distomum sociale, and 

 he includes the genus in the subfamily Dicrocoeliinae Looss, 

 though, at the same time, he points out that in many respects this 

 species approximates closely to the sub-family Brachycoeliinae 

 Odhner [nee Looss). A year later Johnston (1912, pp. 

 329 — 341, figs. 13, 14, 15, 69 — 76) described three more species 

 {Mesocoelium mesembrinum, M . megaloon, and M. oligoon), which 

 he refers to this genus and all of which he found inhabiting the 

 intestine of certain species of Atmra. On p. 340 he gives in 

 a tabular form the main features of this genus and of both 

 sub-families for the purpose of comparison, and he arrives at 

 the conclusion (p. 336) that "amongst known distomes, it is 

 to Brachycoelium crassicolle R. that the four species of this 

 genus appear to be most closely related" ; while on p. 338 he 

 admits that " of all the Brachycoeliinae, the various species of 

 Mesocoelium appear to approach more closely to the Dicrocoeliinae 

 than any others of their subfamily." In many respects Mesocoe- 

 lium appears to be a connecting link between the two subfamilies : 

 as regards the spiny integument, the relative sizes of the oral 

 and ventral suckers, and the habitat of the various species, i.e. 

 the intestine of amphibia, Mesocoelium agrees with the diagnosis 

 of the Brachycoeliinae ; on the other hand in respect of the situa- 

 tion and arrangement of the genital organs, the position of the 

 ovary behind the testes, the juxtaposition of these latter to the 



