DEVELOPMENT AND POSITION OF THE HEAD. 363 



only subsequently to originate; it begins from the first, as in the 

 genuine forms, to become the head. 1 When the neck is formed the 

 elevation begins to take place ; the lower end of the sac, which has 

 become the head, is pushed in like a plug into the other portion. 



This pushing in of the head is a constant characteristic of these 

 bladder-worms, and is not merely occasional, as may be established 

 from a consideration of some features in the organization. In the 

 Cysticercusfasciolaris of the mouse, in which the same mode of formation 

 was observed, we referred the protrusion of the head to the consider- 

 able elongation of the connecting part ; in Cysticercus arionis it is pro- 

 bably the elongated proboscis-like portion which bears the hooks that 

 determines the protrusion. This supposition is confirmed by the fact 

 that in all the other Cysticercoids with an elongated proboscis the 

 same position and attitude of the head are found to obtain. Thus I 

 find it, for example, in a Cysticercoid from the liver of Lymnceus pereger, 

 which closely resembles the Cysticercus arionis in its whole organization, 

 and which, from the structure of its hooks, may probably be referred 

 to the Tcenia microsoma of the wild duck. So is it also with the 

 Cysticercus of Tcenia gracilis 2 found by v. Linstow among the remains 

 of small crustaceans in the intestine of a young perch, and in the 

 Cysticercus lumbriculi found by Eatzel in Scenuris variegata* which 

 probably becomes T. crassirostris in the intestine 

 of the snipe and other water-birds. The Cysticer- 

 coid forms of the Tcenice of the shrew (T. pistillum 

 and T. scutigera T) lately described by Villot from 

 Grlomeris, 4 * are also closely connected with Cysti- 

 cercus arionis in the nature both of its head and of 

 its rostellum. 



There are, on the other hand, also Cysticercoids 

 in which the rudiment of the head originates and FIG. 210. Cysti- 

 grows in the typical bladder-worm fashion which 

 we have formerly described. There are, as we 

 should expect, forms with short rostellum, such as Nordmann's Gypo- 



1 Stein is the only investigator who has observed the first developmental stages of a 

 Cysticercoid. Speaking of this process he says "The further changes of the encysted 

 embryo are as follows : an ever-deepening depression is formed at the truncated anterior 

 end, and the head, with its proboscis and suckers, is at the same time formed in the 

 centre of the embryonic body from the absorbed ground-substance, "Zoc. cit.,p. 210. The 

 accompanying figures show in Tab. x., Fig. 13, a Cysticercus with invaginated portion 

 and rudiment of head, whose hooks are hardly raised as far as the middle of the bladder 

 body, while they are afterwards found close behind the point of invagination. 



2 Archivf. mikrosk. Anat., Bd. xxi., p. 535, 1871. 



3 Archivf. Naturyesch., Jahrg. xxxiv., Bd. i, p. 147, 1868. 



4 "Migrations et metamorphoses des Te"nias des Musaraignes," Ann. Set. nat., ser. 

 6, t. viii., Art. 5, 1878 ; also Ann. Mag. Nat. Hist., ser. 5, vol. i., p. 258. 



