68 ANIMAL PARASITES. 



mature tape-worm (especially in Tetrarhynchi and other species 

 of Cestodea), as it has gradually turned itself over from behind, 

 by which it rises more and more in the bottom of the vertical 

 cavity, so that in opaque preparations it appears as if the head 

 were formed by the elevation of a proper mass. Leuckart thinks 

 that this apparent elevation may perhaps partly arise from the 

 different form of the rostellum in true Cysticerci and the allied 

 forms. 



The heads of bookless Tanice may be developed from the eggs 

 in the same way as the forms just mentioned, but, as a matter of 

 course, with the omission of the hook-formation. The most 

 characteristic point in these cestoid scolices, would be the forma- 

 tion of the sucking apparatus. 



Although certain Tetrarhynchi exhibit an isolation of the 

 head, this may also be often produced originally in the above 

 manner, and the isolation be a secondary phenomenon. Leuckart 

 compares the occurrence of these forms with the phenomena 

 presented by Echinococci. It appears to me that these forms 

 may be best understood by a comparison with Stein's cestoid 

 worms from Tenebrio molitof (Sieb. and Koll. Zeitschr., iv, Taf. 

 20, fig. 12 — 14). Thus, regarding the vesicular caudal ap- 

 pendage as proper to the Cestodea, as we have done, if we sup- 

 pose the constriction placed behind the body to advance so far 

 as to produce a complete separation, Ave have a scolex enclosed 

 in one segment of the embryonic body ; and by this division an 

 isolated caudal appendage (caudal vesicle), and an isolated head 

 with its envelope. It is also possible, in the last place, that there 

 may be embryonal vesicles, which do not become larger than is 

 necessary to form and enclose a single scolex in their interior; 

 the walls of which, therefore, approach so closely to the scolex as 

 almost to touch it, so that there is verv little fluid between them. 

 Of the existence of these latter forms, Van Beneden has furnished 

 numerous proofs ; the separation by constriction is supported by 

 Zeder's observation on a cestoid worm of the pike. 



Peculiar conditions are exhibited by the Echinococci, of 

 which we may undoubtedly distinguish two great groups, to 

 which I have given the names of Echinococcus altricipariens, and 

 E. scolicipariens. In them, even more than in the Ccenuri, the 

 passive function of the residue of the embryonal vesicle as caudal 

 vesicle falls into desuetude, and its independent morphological 

 destination as a mother-vesicle becomes still more prominent. 



