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KEPOKT ON ZOOLOGY, MDCCCXLIV.

to the position of the male sexual organ ; in some Bra -chylaimi for instance, the male organ opens in front ofthe ventral acetabulum; in others, on the contrary, it doesso in the middle between the ventral acetabulum and theextremity of the abdomen, whilst the oviduct opens ante-riorly and externally. At the extremity of the abdomenof these Trematoda a pore leads into a contractile cavity,from which four winding, mutually-anastomosing, lateralcanals proceed upwards, which are here and there furnishedwith solitary undulating cilia. The Eeporter in thisdescription recognises the excretory organ which is peculiarto the Trematoda, as well as the vascular system providedwith ciliated lobes, which Dujardin has not distinguishedfrom that excretory organ. Besides those taken from themale genital organs, Dujardin also finds in Brachylaimusvarious other specific characters, which relate to the size ofthe acetabula and of the ova, and to the greater or less prick-liness of the cutaneous surface. The same naturalist hasreferred to this genus a Distomum found in Sorex araneus,as Brachylaimus advena; but has named it afterwards (inhis larger work on the Helminthes) Brachylaimus migrans.He derives this animal from the Distomum inhabiting theLimax, which presents exactly similar acetabula, a similarintestinal canal, and the same system of vessels furnishedwith vibratile cilia, and in which the male genital apparatusexhibits a form, in all respects similar; whilst, of the femalesexual organs, nothing is as yet to be recognised. SimilarDistomata in Progs are probably derived, in Dujardin’sopinion, from Snails that have been swallowed.

According to Kolliker ’s observations (Muller’s Archiv ,1843, p. 99), the ova of Distomum tereticolle are thusdeveloped: in the midst of the compact granular vitellinesubstance the primitive embryonic cells are formed, increase,break through the vitellus, and entirely consume it. Thecollective mass of the very minute embryonic cells isfinally transformed into the body of the embryo. This isof a vermiform shape, moves sluggishly, exhibits no trace of