66 A FURTHER KNOWLEDGE OP THE CYSTIC CESTODES, 



worm, are furnished with a distinct though small rostellum and 

 with the rudiments of hooks" ; and further Nitsche, according to 

 Leuckart,* has occasionally observed, in the adult form, these 

 rudimentary hooks. In view of these facts, Villot's opinion, in 

 so far as it rests on the case of T. sagiriala, loses its value. 



It may now be well to institute some comparison between the 

 Cysticerci properly so-called and the two forms described in the 

 foregoing as re|)resenting the genus Piestocystis. 



The unarmed Piestocystes are not only sharply marked off from 

 the Cysticerci properly so-called by the absence of hooks, but also 

 by their general structure. The retractile more or less conical 

 head, destitute of hooks and provided with four large and well- 

 marked suckers, is characteristic, as also is the elongated posterior 

 portion usually interpreted as representing the caudal vesicle. 



According to Villot, four parts can be distinguished in an 

 ordinary Cysticercus : (1) the cyst, ("2) the head, (3) the body, (4) 

 the caudal vesicle. The cyst is in the Cysticerci pro[)erIy so-called 

 an adventitious structure derived from the host, and having no 

 genetic connection with the enclosed Cysticercus, and so may be 

 left out of consideration in the present connection. The Cy.sti- 

 cercus itself consists of the three parts — head, body, and caudal 

 vesicle. If we, with Leuckart and other observers, consider the 

 posterior part, in which the head is invaginated, in these unarmed 

 forms, as representing the caudal vesicle, what corresponds to the 

 body of the ordinary Cysticercus % It seems to be represented by 

 the muscular sheath which immediately surrounds the invaginated 

 head and which has in the foregoing been regarded as comparable 

 to the " receptaculum capitis " of Leuckart. The " receptaculum 

 capitis" of Leuckart is, as Villot has shown,! formed by tlie 

 internal of the two layers into which the somato-cephalic bud of 

 the developing Cysticercus separates. From the internal layer 

 (the receptaculum capitis) the inner wall of the body is derived, 

 while from the external are derived the outer wall of the body 



* Loc. cit. p. 435. 

 t Loc. cit. p. 14. 



