374 THE DEVELOPMENT OF CESTODES. 



Wagener has made of it (see especially Fig. Ill of his work) leave 

 not the slightest doubt on this point. According to the description 



of van Beneden, a similar appearance is 

 found in the Cystieercus of Acanthdbothriwm 

 coronatum. The cavity of the suctorial 

 cups is always directed towards the interior 

 of the invaginated cavity, and the future 

 crown is situated meanwhUe as far back as 

 possible on the rudimentary head. 



In contrast to this, however, the head 

 of Tetrarhynchus, &c. is said not to originate 

 directly from the first sac-like structures, 



but to be subsequently developed in the 

 FiQ. 218. — Larva state of . . . , ,, , 

 Acantkobothnum coi-onatum, interior, by the base rising up into a 

 after van Beneden. ( x 25.) thimble -like projection, inside which the 

 different organs of the head are formed. " If we imagine the 

 thimble-like projection from the base of the sac broadened out above 

 like a mushroom, we have the head of a Dibothrium, which might be- 

 come a " dibothrian " Tetrarhynchus by the addition of proboscides." 

 So we read in Wagener, ^ and van Beneden says pretty much the same 

 thing, but differs in so far as he takes the Scolex-iorm for his starting- 

 point (see p. 329), and does not regard the head-sac as a new struc- 

 ture, but the retracted anterior end of the worm. 



After the foregoing observations, it cannot be doubted that in 

 the Tetrarhynchi and the related forms the base of the originally 

 quite simple^ head-rudiment is raised into a boss-like elevation to 

 effect the formation of the head proper. But of course that does not 

 imply that these plugs alone produce the head. This is by no means 

 proved by the preceding statements, for these all rest upon investiga- 

 tions which are quite insufficient to establish any such conclusion. 



In order to study more especially the process of the formation 

 of the head, I have investigated by means of sections a number 

 of young Tetrarhynchus-hladdevs from the muscle of LopMtis pisca- 

 torius. The material certainly furnished me with no continuous 

 developmental succession, but it convinced me most distinctly that 

 the elevation only takes place at a time when the suctorial cups 

 and proboscides are already formed, and when the head, with its 

 different parts, is thus essentially mature. Upon the whole, the 

 conditions are quite similar to those of the typical bladder-worms. 



' Loc. cit, p. 62. 



^ In claiming this rudimentary head as simply a hollow bud, I do so not only on the 

 ground of Wagener's statements, but on the basis of my own investigations of Tetra- 

 rhynchus and Echinobothriu&jgjtjzed by MicrOSOft® 



