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dilated, at their posterior extremity ; and the anterior end may be 

 retracted into the sac thus formed, which then invests it Uke a double 

 serous sac — a structureless investment may be excreted round this 

 encysted worm or it may not. Such an altered Cestoid Worm as this 

 is called a Cysticercus. 



2ndly. A dilated Cestoid worm, such as has been just described, 

 may develope new " heads " with suckers and hooks all over its outer 

 surface, never developing any upon its inner surface. Such a Cestoid 

 worm is the Cceniirus cerehralis. 



Srdly. The Cestoid worms all possess the power of gemmation (or 

 it may be called fission) in their unaltered state : and Bendz (Isis, 

 1844) has distinctly shown that the vesicular extremity of the Cysti- 

 cercus gemmates. Processes are formed and thrown off, and these 

 develope appropriate heads and hooks, becoming complete Cysticerci. 

 Bearing these facts in mind, it is I think very easy to account for 

 the Eckuiococcus-yesides. The surfaces which produce the Echino- 

 cocci must be both external; the Echinococcus-cyst therefore does 

 not answer to the simple cyst of the Coenurus, or of the protruded 

 Cysticercus ; but to the double cyst of the retracted Cysticercus, the 

 upper half of whose proper outer surface forms the inner wall of the 

 cyst in the retracted state (see Diag. D. PI. XXIX.). 



Suppose the cyst, thus formed, to dilate and to develope a multi- 

 tude of heads upon this upper half of the outer surface, after the 

 analogy of Coenurus : then the two walls being pressed together into 

 one, it will appear like a simple cyst covered with heads internally 

 (Diag. E.). 



If, however, at the same time, in complete correspondence with 

 Coenurus, heads have been developed over the whole outer surface, 

 we have the primary Echinococcus endocyst (Diag. F.). 



Now the cyst may grow out at a particular point, and so form a 

 bud, which is cast off externally. This takes place in the Echino- 

 coccus of Oxen. But if it have surrounded itself with a dense cyst, 

 analogous to that of the encysted TetrarhynchidcB, such external 

 budding cannot take place ; and if the local growth takes place at all, 

 it will produce a projection internally, and the internal fixed second- 

 ary cyst will be produced. These, narrowing at the neck and de- 

 taching themselves, become the free secondary cysts as was shown 

 above. 



The Echinococcus then is a species of Tcenia which has become 

 dilated and encysted ; which has subsequently produced heads all 

 over its external surface, and finally, budding, casts off its vesiculai* 

 processes internally, because it has no exit for them externally. 



Echinococcus is thus the most complex form of that change which 

 young Cestoid Worms are liable to undergo if they wander from 

 their proper nidus ; the combination of hooks with suckers refers it 

 to the genus Tcenia, to which Coenurus and Cysticercus may by similar 

 reasoning be shown to belong ; and, therefore, like these two latter 

 genera, it must, as a genus, be abolished. It is probable however 

 that Cysticercus, Coenurus and Echinococcus are modifications of 

 distinct species, or groups of species, of the genus Teenia ; and are 



