152 PRACTICAL PAEASITOLOGY 



the head is a more difficult matter and is only to be carried out by 

 means of sections in unbroken series. 



Examination of Cysticercoid Stages. 



The , Cysticercoid stages (Cysticercus tenuicollis) of the Tcenia 

 marginata of dogs, which undergoes development in the omentum of 

 sheep, are readily obtainable from abattoirs. On account of their 

 large size, they are very suitable subjects for a first examination of the 

 structure of Cysticerci. When fresh, the Cysticercus is generally still 

 enclosed in the cyst, derived from the tissues of its host, which must 

 be opened with great care to avoid damaging the inmate. Attached 

 to the body of the Cysticercus is a chalk-white, cone-shaped formation, 

 which projects in a varying degree above the surface of the bladder, 

 and at the free end of which an orifice, into which a bristle may be 

 inserted, is situated. This projection is the head-cone. It is a hollow 

 organ with thick walls which are continued inwards, and it contains 

 the head parts of the future tapeworm (suckers, rostellum), which 

 develops inwards within the hollow. As soon as the Cysticercus is 

 imported into the intestine of the definitive host, namely, the dog, the 

 head-cone becomes completely everted and forms the scolex of the 

 future tapeworm, the bladder at the same time perishing. In Cysti- 

 cercus tenuicollis, as in many other species, this process of evagination 

 frequently begins in the intermediate host, and it may be completed 

 by artificial means in the isolated specimen. The parasite should be 

 held just below the head-cone in the finger and thumb of one hand, 

 while a regular pressure from within outwards is exerted with the 

 fingers upon the head-cone. The head-cone will lengthen and, if the 

 manipulation is repeated several times, it will turn inside out and will 

 hang down as a flat, wrinkled, contractile band upon the surface of 

 the bladder. If pressure is now exerted upon the free end, the head 

 will finally become everted, springing out with a sudden jerk. It will 

 appear as a round body of considerable transparency (owing to the 

 absence of calcareous bodies) at the end of the chalk-white band of the 

 everted head-cone. It is not easy to examine the head in this condition 

 because, as soon as pressure is removed, it is usually again retracted. 

 The head should, therefore, be cut off from the head-cone, the peripheral 

 end being laid upon a glass slide, and pressure should be exerted upon 

 the cut end until the head again appears, when a cover-glass should 

 immediately be placed upon it. It is frequently impossible to extrude 

 the head completely, the rostellum remaining partially inverted. In 

 this case the hooks, with which the edge of its summit is crowned, will 

 have their points directed upwards instead of sideways. Successful 

 specimens may, however, be obtained, and these should be fixed 



