676 



DESCRIPTION OF THE BOTHRIOCEPHALID^. 



animal, in which the various organs and functions, formerly distributed 

 over different individuals, are united in one (p. 386). 



The slighter individualisation of the different parts of the body 

 also finds expression in the processes of development. Instead of the 

 former features of an alternation of generations, we find in the 

 Bothriocephalidae modifications, which rather suggest a simple meta- 

 morphosis. The primary stages of development disappear, and there 

 is a corresponding diminution in the difference between the larva 

 and the adult worm. The larval form becomes like the sexually 

 mature animal, and sometimes to such a degree, that they can only be 

 distinguished by the imperfect development of the sexual organs in 

 the former. This is perhaps most striking in the Ligulidse (Schisto- 

 wphalus, Ligula), which in their intermediate hosts, the stickle-backs 

 and bleak (Leuciscus), not only grow to their full size, but form their 

 sexual organs, which develop so far that, after the transference into 

 the intestine of their definitive host (a 

 piscivorous bird), maturity is attained in 

 an exceedingly short time (p. 379). In 

 Archigdes the whole development takes 

 place in the intermediate host. So far as 

 we know (p. 378), it is the only tape-worm 

 that undergoes no change of host, and that 

 never loses the characteristics of its larval 

 life. 



These are, however, mere exceptions. 

 Usually, the larval forms of the Bothrio- 

 cephalidae are easily distinguished from the 

 adult worms, not only because they are 

 destitute of reproductive organs, but also 

 because they are unsegmented (Fig. 354). 

 They possess, as a rule, a longer or shorter, 

 ribbon-like body, whose anterior end bears 

 the suctorial grooves, and is thereby re- 

 cognised as the head. 



It is doubtful whether the ribbon-shaped 

 posterior part of the body is always cast off, as in the 

 Cysticerci, on the transition to the final state ; and that 

 the more, since the head of these worms hardly ever 

 attains its full development and size during the larval 

 life, but only after the transference to the alimentary 

 canal of the definitive host. 

 The Suctorial Grooves in the larval forms are often shallow and 

 indistinctly limited in comparison with the subsequent general pro- 



FIG. 354. 

 Larva of .60- 

 thriocepkalus 

 ( x 55.) 



FIG. 353. 



Archigetes 



Sieboldi. 



( x 60.) 



