STRUCTURE OF THE PROGLOTTIDES. 525 



those remarks. 1 Whether there are also adult tape-worms with mal- 

 formed hooks is doubtful, since the imperfect development of the 

 hook-apparatus would hardly permit a long enough residence in the 

 intestine to allow of the full development of the worm. 



The Structure of the Joints calls for little addition to our previous 

 description, except as regards the sexual apparatus. Everything else 

 has already been mentioned in one place or other. We know, for 

 example, that Tcenia solium is far from possessing the powerful mus- 

 culature of T. saginata. We are also acquainted with the fact that 

 the four longitudinal vessels originally present are reduced to two 

 before the appearance of the sexual organs, and that they then run 

 throughout the whole length of the chain in the form of two wide 

 stems, which are always connected at the posterior border of each 

 joint by a transverse anastomosis. It is true that for a time the four 

 stems may be seen near each other, but they are no longer arranged 

 in pairs on the two flat surfaces, as in the head, but are all on the 

 same plane and close together. The change of position takes place 

 at the beginning of the neck, and is evidently caused by the marked 

 flattening of the segmented body, which no longer admits of the former 

 distribution of the vessels. In consequence of this, each of the two 

 narrow sides has an outer and an inner vessel, so arranged that 

 the vessels of the ventral female surface become the external, while 

 those of the dorsal male surface come to lie internally. It is difficult 

 to say by what influences the two inner vessels have become dwarfed, 

 but this is very probably due to the peculiarities of the position of 

 the worm. Moniez conjectures that the cause of the diminution is 

 the development of the sexual organs, which might, indeed, have some 

 effect on the adjoining parts ; but it is demonstrable that they only 

 attain their complete development at a time when these vessels have 

 already disappeared. 



In regard to the calcareous corpuscles, we need only remark that 

 they are upon the whole sparsely distributed, and less marked than in 

 the related species, especially T. saginata. It appears, however, as 

 though all specimens were not alike in this respect. They also 

 exhibit many variations in size, but, as above mentioned, seldom ex- 

 ceed 0-012 mm. 



The Reproductive Organs exercise a marked influence upon the 

 general form and organization of the body, just as in Tcenia saginata 



1 In support of the assertion which I made at that time, that there are some bladder- 

 worms in which the circlet of hooks is not developed, I refer to the case described by 

 Lewin (Annalen des ChariU Krankenhauses, ii., p. 167 : Berlin, 1875), who found a bookless 

 bladder- worm from a pig, which possessed, instead of the rostellum, and evidently as a 

 representative of it, " an extra fifth sucker." 



