386 Zoological Society. 



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

 retracted into the sac thus formed, which then invests it hke 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 Ccenurus cerebralis. 



3rdly. 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 Echinococcus-y tsicXes. The surfaces which produce the Echino- 

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

 not answer to the simple cyst of the Ccenurus, 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. XI.). 



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 Ccenurus ; 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 

 Ccenurus, 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 Echitio- 

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

 analogous to that of the encysted TetrarhynchidcBy 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 secondary 

 cysts will be produced. These, narrowing at the neck and detaching 

 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 vesicular 

 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 hable to undergo if they wander from 

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

 to the genus Tisnia, to which Ccenurus 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, Ccenurus and Echinococcus are modifications of 

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

 not mere varieties of one species produced by difference of locality. 

 They are all three found in the brain, for instance. 



