SIGNIFICANCE OF THE HEAD. 



273 



As the often corroborated observations of these investigators showed, 

 there is in the developmental history of the Cestodes a stage in 

 which nothing of the future tape- 

 worm is to be seen, except the so- 

 called "head," which then lives by 

 itself like an independent animal, 

 and only gradually and under favour- 

 able conditions buds out one joint 

 after another from its posterior ex- 

 tremity. This isolated tape-worm 

 head, or more correctly " nurse," 

 had indeed been formerly observed, 

 but its true value was not de- 

 tected, and it was usually regarded 

 as the representative of a special 

 form (Scolex, &c.). 



The joints, which are budded 

 off one after the other, are like all 

 buds, at first small and immature, 

 but they gradually increase in size 

 and maturity as they are pushed 

 further from the head by the in- 

 tercalation of new buds. After a 

 certain time, or, what cornes to the 

 same thing, at a certain distance from their nurse, they become sexu- 

 ally mature, sooner in some species than in others. The head, which 

 is usually designated by the originally much more restricted name of 

 " Scolex," remains, like a true nurse, asexual. 



Thus by continued budding there arises from an originally isolated 

 nurse a whole community of individuals a colony, in which one 

 has to distinguish not only animals at various stages of maturity, but 

 also one asexual and aberrant member, the so-called " head." 1 



The significance of this head is not, however, purely genetic. 

 It is not only the origin of the whole chain, but serves further for 

 its fixation, and thus benefits the whole colony. It was formerly be- 

 lieved that nutrition was also effected by the head ; 2 the suckers were 

 regarded as mouth openings (Nitzsch, Owen), or the oral opening was 



1 Van Beneden used the term " strobila " to designate this colony. It was formerly 

 employed by the famous Norwegian zoologist, M. Sars, to denote a stage in the development 

 of certain jelly-fish (Aurelia) a stage, indeed, formerly described by him as a new species. 

 In this instance we have, as in the tape- worm, a "nurse " here in the form of a polype, on 

 which there rises a pillar of young jelly-fish, at first imperfectly separated from one another, 

 but afterwards set free. See Archiv f. Naturgesch., p. 9, tab. i., 1841. 



2 As we shall afterwards see, a similar opinion has lately been advanced by Blumberg. 



S 



Iso- 



; head" 



of Echineibothrium 

 minimum, van Bene- 

 den, from the intes- FIG. 134. Chain 

 tine of Trygon pas- of joints of Echinei- 

 tinaca. bothrium minimum. 



