BOTHRIOCEPHALIDiE. 127 



smaller, sometimes being scarcely visible. Whilst tlie scolex is still 

 witliin tlie mouse tlie larva frequently assumes tlie teenioid condi- 

 tion ; but in all sucli cases, the incomplete deyelopment of the 

 sexual organs shows that the parasite is still a larva, and has not yet 

 gained access to its proper ultimate host. In most Cysticerci 

 there is usually no trace whatever of the future reproductive appa- 

 ratus of the tapeworm. 



Amongst the few other cestode larvse which have been satisfac- 

 torily traced to their adult strobila condition, I need here only 

 mention the common Gysticercus cellulosfB of the pig, which becomes 

 the Tcenia solium of man, the Gysticercus of the calf and ox, which be- 

 comes the Tisnia mediocanellata of man, and the Gysticercus longi- 

 collis of the meadow vole, which becomes the Tcenia crassiceps of 

 the fox. Full details of the structure and development of the 

 two former will be found in the second part of this work ; but 

 the genetic relations subsisting between most other known tape- 

 worms and their larvae have not yet been satisfactorily demon- 

 strated, although there cannot remain a shadow of doubt that the 

 developmental process is essentially the same throughout all the 

 numerous members of this singular class of parasites. 



BothriocephalidcB. — In this great family I include all those 

 cestodes which have their heads furnished with two or four opposed 

 bothria or fossettes, whether these latter exist merely in the form 

 of shallow pits or grooves, or whether they are developed into 

 leaf-like appendages more or less prominent. A considerable 

 number of the species are armed with hooks, which are usually 

 placed at the upper part of the fossse. All of them present 

 elongated, depressed, and jointed bodies, but in a few forms the 

 segments are not very clearly defined. The head varies much 

 in the different genera, being usually conspicuous and much 

 broader than the upper part of the neck. In some cases it is 

 flattened and oval in outline, frequently truncate at the top, 

 occasionally quadrangular, and very commonly spread out in the 

 form of a tetrapetalous flower. The mature proglottides vary 



