FORMATION OF THE HEAD OF TETRARHYNCHUS. 



375 



FIG. 219. Longitudinal sec- 

 tion of a still imperfectly de- 

 veloped Tetrarhynchus from the 

 muscle of Lophius. ( x 25.) 



As in these, the suctorial cups at first lie in the wall of the rudimentary 

 head, close in front of the posterior caecal end, so that their cavity 

 is directed inwards. Between them there 

 is as yet but a slight elevation, which 

 scarcely rises to the height of the suctorial 

 cups, and covers itself with the lower 

 muscular borders of the latter. On the 

 anterior borders of the same may be 

 noticed the opening of the proboscis- 

 sheaths, which extend back along the floor 

 of the caecum with their invaginated 

 hooks and terminal pouch, and which are 

 externally covered by the receptacle-like 

 muscular sheath surrounding the rudi- 

 mentary head. At the anterior border 

 the fibres of this sheath may be observed 

 bending round into the peripheral mus- 

 culature of the Cestode-bladder, which is filled inside with large 

 granular cells. 



We thus get the same result in regard to the Tetrarhynchi as we 

 formerly did from our investigations of the Tcenice, and they may be 

 summed up thus, that it is the sac-like invagination of the bladder 

 itself which produces the head. The elevation always appears only as 

 a secondary structure of subordinate morphological importance, and 

 is, moreover, by no means so widely distributed as the statements of 

 some investigators would lead one to suppose. 



All the Cestodes which we have as yet considered, in spite of their 

 other differences, agree in being provided with a well developed 

 " head," which is in form and structure in sharp contrast to the rest 

 of the body. But, besides these, there are a number of Cestodes with 

 a simple, more or less inconspicuous head, which is as a rule provided 

 with two superficially situated longitudinal pits, a circumstance 

 which has procured for these animals the name of " Dibothria." Like 

 the head, the segments are less individualised, being not unfrequently 

 indicated only by the successive repetition of the sexual organs 

 (Ligula, Tricenophorus), or even represented by a completely simple 

 body (CaryopJiyllceus, Archigetes). The genus Bothriocephalus (sensu 

 stricto) is the first of this group, the only tape- worm outside the genus 

 Tcenia which is found in man. 



Such being the case, a special interest naturally attaches itself to 

 these tape-worms. All the more is it to be regretted that our experi- 

 ence of the forms in question is hitherto only slight. It is true that 

 we are acquainted with a number of young Dibothria in the higher 



