BY W. A. HASWELL AND J. P. HILL. 



interpretation of Stein's observations, we have an example of a 

 similar form, in which the aperture persists. Examples of the 

 third group are Cysticercus arionis and the Cysticercoid of T. 

 infundihuliforTnis (according to Grassi and Rovelli's account of it). 

 It is to this group likewise that Folycercus is referable, and also, 

 we believe, Villot's Staphylocystis. 



In many Cysticercoids it would appear that there is a progres- 

 sive invagination of the anterior parts within those lying behind : 

 the anterior part of the head is invaginated within the posterior 

 part ; the head is invaginated within the body and the body 

 within the caudal bladder. This condition of things is clearly a 

 secondary one, brought about in adaptation to special circum- 

 stances. The enclosure of the head within the body and caudal 

 bladder doubtless subserves the protection of the former ; while 

 at the same time it doubtless permits of the passage of the parasite 

 to that part of the alimentary canal of the host in which the adult 

 Cestode is capable of living — the hooks and suckers not coming 

 into play for purposes of attachment until the intestinal juice has 

 had the effect of causing evagination of the head and rostellum. 



In Folycercus this adaptation may be said to reach its furthest 

 known limit. Here there is no invagination in the strict sense, 

 but the parts of the Cysticercoid are actually developed one 

 within the other, the head within the body and the body within 

 the caudal bladder, and it is only subsequently, when the Cysti- 

 cercoid is about to pass into the adult Cestode, that by a process 

 of evagination these parts assume their normal and primitive 

 relations to one another. Whether this condition is exceptional 

 or the reverse it would be impossible to say in the present condi- 

 tion of our knowledge of the development of the Cysticercoids. 

 That it occurs in another form we know from the observations of 

 Stein on the development of Cysticercus tenebrionis* ; while we 



* " Beitriige zur Entwickelungsgeschichte der Eingeweidewiirmer." 

 Zeitschr. f. wiss. Zool. iv. Band (1853). Confirmed by Meissner, "Zur 

 Entwickelungsgeschichte und Anatomic der Bandwiirmer." Zeitschr. f. 

 wiss. Zool. v. Band (1854). 



