206 ON THE ANATOMY OF ECHINOCOCCUS VETERINORUM 



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

 the Ec/ihiococci/s-vesicles. The surfaces which produce the Echino- 

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

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

 Cysticerciis ; 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. [Plate 22]). 



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 Ccemiriis : 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 

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

 we have the primary Echiiiococais 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 Ecliino- 

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

 analogous to that of the encysted Tetrarliynchid(£, 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 Ecliinococcits then is a species of Tcstiia 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. 



Ecliinococcits 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 Tceiiia, to which Ccenurus and Cysticerciis 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 Cysticerciis, Ccenurus z-nd Echinococcus are modifications of distinct 

 species, or groups of species, of the genus Tcsnia; and are not mere 

 varieties of one species produced by difference of locality. They are 

 all three found in the brain, for instance. 



As to the genus Acephalocystis, there is good reason for believing, 

 that all genuine specimens of it are Echinococcus-cysis which have 

 either not developed heads, or in which they have been overlooked. 



The converse of the anatomical evidence as to the identity of 



