32 THE INTERNAL PARASITES OP 



both to project and retreat at my pleasure. I have, 

 therefore, not the smallest doubt but that during life 

 the process in question is perfectly capable of being 

 thrust forward and backward at the will of the entozoon, 

 as undoubtedly obtains to a more striking degree in 

 the case of those tapeworms which are armed with a 

 retractile proboscis or rostellum. 



I have purposely dwelt upon the character and 

 functions of the central organ because several able and 

 estimable observers appear to have entirely misunderstood 

 the nature of this structure. In the pages of Nature, 

 indeed, I have already animadverted upon this prevalent 

 error ; but hitherto I have been unable to give adequate 

 expression to the opinion, which is here offered only 

 after a careful and prolonged investigation of the ques- 

 tion at issue. In this relation, also, I have yet further 

 to remark that the head of the pork tapeworm (as shown 

 in the two lower representations) not only presents a 

 quadrilobate figure when seen from above, as at A, but 

 that it also exhibits a distinctly conical figure when 

 viewed in profile, as seen at B. Moreover, the upper 

 part of the rostellum displays a double circle or coronet 

 of hooks, the points of which are seen to project con- 

 spicuously at the circumferential margin of the crown. 



A full and satisfactory view of the beef measle can 

 only be obtained by rupturing the cyst, and then, by 

 subsequent gentle pressure, causing the head and neck 

 to protrude from the receptacle. It will now be seen 

 that the so-called neck, which some entozoologists with 

 equal propriety term the body, presents a series of 

 transverse folds which are correctly regarded as the 

 limitation lines of a set of imperfectly developed segments. 

 These, in the adult tapeworm, eventually become 



