As already hinted, this cestode is very liable to present abnormalities of structure, the proglottides frequently displaying double sexual orifices, with corresponding duplication of the reproductive organs internally. For details respecting the anatomy of _Bothriocephalus_ I must refer to the works of Küchenmeister and Leuckart; and more particularly to the memoir of Drs F. Sömmer and L. Landois, who have supplemented the previous researches of von Siebold, Leuckart, Böttcher, Stieda and others by beautiful investigations of their own. In the pages of ‘Nature,’ for 1872, I gave a _résumé_ of Sömmer’s memoir, which will be found quoted below. _Bothriocephalus cordatus_, Leuckart.--This species is identical with a worm long ago described by Pallas and Linneus. At present it is only known to infest the residents of North Greenland, but it is probably distributed throughout the north generally. It attains the length of about one foot, and has a small heart-shaped head, whose apex is directed forwards. The neck is so obscure that it may be said to be altogether wanting, the segmentation of the body being well marked immediately below the head. Though so small a species, Leuckart, who first described it, counted between six and seven hundred joints. As in the broad tapeworm, the reproductive orifices are serially disposed along the centre of the ventral line, but a close inspection shows that the folds of the egg-bearing organ are comparatively more numerous. This worm does not appear to be a frequent resident in the human body, though it is by no means uncommon in the dog. Possibly it may yet be found in the inhabitants of some of our northern and western isles. [Illustration: FIG. 28.--Head of _Bothriocephalus cristatus_, viewed from the front. After Davaine.] _Bothriocephalus cristatus_, Davaine.--This cestode measures between nine and ten feet in length, and is characterised by the presence of two remarkable prominences, together forming a sort of rostellum or crest which is covered by numerous minute papillæ. The full-grown segments are less than half an inch in breadth; the body of the parasite being narrower than that of the broad species. The original description of the parasite by Davaine is based on two specimens, one of which, quite perfect, was obtained from a child five years old, under Dr Féréol’s care at Paris. The other was passed spontaneously by an adult residing at Haute-Saône. I have here copied one of Davaine’s original figures of the head of the worm.