20 ON PARASITES. 



two small longitudinal excavations in the solid tissues of these ani- 

 mals terminating anteriorly in the minute pore in the centre of the 

 head and united by cross excavations in the joints. The heads of 

 the cestoidea being so constantly armed with booklets and sucking 

 discs they must be designed to fasten upon the intestinal mucous 

 membrane and absorb therefrom a portion at least of their nourish- 

 ment. 



Five metamorphoses are observed in this order : — 1. The mature 

 or perfect animal (proglottis). 2. The hooked embryos. 3. The 

 resting scolex, appearing, {a) with vesicular appendages ; (b) a hand 

 like appendage ; (e) no appendage. 4. The active scolex. 5. 

 strobila. 



Family I. Bothbiocephali. 



These animals are furnished with two lateral depressions or suck- 

 ing discs upon the head, which is more or less tetragonal. The de- 

 pressions are usually naked. The head is obtusely conical. 



The atrobila has a dorsal and ventral surface. Four margins are 

 defined on each segment, — the two lateral free, — the anterior and 

 posterior unite the segment to its anterior and posterior neighbors. 



The genital pores are situated in the mesian line. It is most 

 common in Russia and Poland. JSTumerous other members of the 

 family have been described, but as they do not occur in the human 

 body, and their cystic worms have not been discovered, I omit further 

 allusion to them and proceed to the 



Family II. TiENiJE. 



This family is a very extensive one, finding its especial habitat in 

 fishes, the perfect animal being most abundant in the predacious 

 ones. It occurs also in piscivorous raptorial birds. Among mam- 

 mals it occurs to a certain extent when they live on the sea shore at 

 the Horth. Those living inland are exempt, except man, in whom 

 only is found the Taenia solium. He being omnivorous, there is 

 a strong presumption that he introduces along with his marine food 

 the scolices of this parasite. 



The experiments of Eschricht seem to prove that a species of 

 ligula is one stage of its development. This ligula is found in large 

 quantities in the flesh of the dorse and other fish inhabiting the 

 northern seas. 



