THE CYSTIC ENTOZOA. 55 



never grows in the same proportion, is very minute, which seems 

 to render probable the notion that the caudal vesicle of these 

 cestoids has not originally been a scolex receptacle, but is rather 

 the hinder end of the tape-worm in a diseased and dropsical 

 state. Such dropsical enlargements are undoubtedly found occa- 

 sionally in single joints of the Cesloidca ; but if attention is paid 

 to the gradual development of the tsenioid Cysticercus fasciolaris 

 it will be seen that its caudal vesicle is at an early period a true 

 receptaculum scolicis. I have before me many examples of the 

 Cysticercus fasciolaris which present the most various stages of 

 development, the oldest being from five to seven inches long, 

 whilst the youngest were no more than from one to four lines. 

 Amongst the older individuals the long body is distinctly jointed; 

 in the younger the short body merely presents very close trans- 

 verse wrinkles as indications of the future joints. At all these 

 different ages, the caudal vesicle appears to have about the same 

 size, and indeed in the older individuals is even somewhat smaller. 

 The youngest individuals, of one and one and a half lines long, 

 have no body at all ; in these the head and neck of the scolex 

 only project, in the slightest degree, from the vesicular recep- 

 tacle, whilst these parts, in the smallest individuals, which possess 

 a perfectly rounded vesicle, are still, as I have most distinctly 

 convinced myself, concealed in the receptacle. If one were care- 

 fully to inspect the livers of many different species of murine 

 rodents, one would certainly come upon still younger forms of 

 development of the scolex of Tcenia crassicollis ; one might even 

 be so fortunate as to discover the embryo hooks on the external 

 surface of the receptacle of the developing scolex, although the 

 discovery of these six hooks of the cestoid embryo, on account of 

 their excessively small size, is a very difficult task. In the Cys- 

 ticercus pisiformis, which, always encysted, infests the livers of 

 hares and rabbits, I have succeeded in meeting with the scolex 

 receptacle, in a very early period of development, with a diameter 

 of half a line. The internal gemmation had just begun, the four 

 suckers of the future scolex were scarcely sketched out, and the 

 still soft apices of the circlet of hooks were only just in process 

 of formation (see fig. 33, d, e), but I sought in vain for the six 

 embryo-hooks on the external surface of the receptacle. In 

 pursuing these inquiries, however, a very interesting phenomenon 

 was presented by the livers of some wild rabbits of this neighbour- 



