SECT. v. ENTOZOA. 145 



suckers and a circle of booklets round a terminal pro- 

 boscis to attacb it to its victim. Tbougb destitute 

 of tbe organs of nutrition it is extremely prolific, for 

 eacb segment of its long flat body is a reproductive 

 monoecious zooid, wbicb forms and lays its own eggs 

 exactly as if it were a single independent animal, tbus 

 furnisbing a very remarkable instance of tbe law of 

 irrelative repetition, wbicb is a series of organs per- 

 forming tbe same functions independently of one an- 

 otber. Two pairs of canals containing a clear colourless 

 liquid extend tbrougbout tbe body of tbe worm. 



Bags, or vesicles called cysts, bad been found in tbe 

 glands and muscles of various animals, afterwards dis- 

 covered to contain young worms, wbicb attain tbeir 

 perfect development witbin sucb creatures as eat tbe 

 flesb containing tbe cysts. Under circumstances so un- 

 precedented, it required no small skill and patience to 

 determine tbe life-bistory of tbese singular creatures. 

 Tbe cysts differ in size and form according to tbe genera, 

 and are embedded singly or in groups in tbe flesb of 

 tbeir victim, on wbose ready prepared juices tbey live. 



Tbe greater number of tbe Tsenia genus begin tbeir 

 lives as sexless cysted larvae, and on entering tbeir final 

 abode, segments are successively added till tbe worm 

 bas arrived at its adult state. Tbe tape- worm of tbe 

 cat bas its origin in tbe encysted larvae found in tbe 

 livers of tbe mouse and rat. One species of Entozoa, 

 wbile in its primary state, inbabits tbe stomacb of tbe 

 stickleback, and only comes to perfection witbin tbe 

 aquatic birds tbat feed on tbis fisb. Another species 

 infests tbe livers of tbe salmon tribe, and gets its per- 

 fect form in tbe pike and percb. 



Sbeep and tbe bog are more tormented witb cysted 

 worms tban any otber domestic animals used for food. 

 If introduced into tbe buman intestines by eating raw 

 bam or sausages, tbe larvae soon acquire tbe perfect 



VOL. II. L 



