270 CHARACTERS OF THE PLATODES. 



as in the Hirudinea, it has existed for a time, it has been obliterated 

 in the course of further development. Under such circumstances 

 the musculature of the viscera only rarely acquires any inde- 

 pendence, and is generally closely connected with the musculature of 

 the body. However, this is not equally true, as we shall presently 

 see, for all Platodes. Nor is the disappearance of the body-cavity 

 always quite complete, for some of the higher, and even some of the 

 lower (parasitic) forms show evident traces of this structure. 



ORDER I. CESTODA. 



Van Beneden, " Les vers Cestodes ou Acotyles," Mm. Acad. roy. Belg., t. xxv. : 

 Bruxelles, 1850. 



Idem, " M&noire sur les Vers intestinaux," p. 112 : Paris, 1858. 



Flat-worms without mouth or alimentary canal, which typically de- 

 velop by alternation of generations, by budding from a generally pear- 

 shaped nurse, with which they remain united for a lengthened period as 

 a ribbon-like colony or ' strobila" The individual joints of the colony, 

 i.e., the sexual animals or Proglottides, increase in size and maturity as 

 they are removed further from their origin, by the intercalation of new 

 buds, but are not distinguished in any special way. The nurse, how- 

 ever, known by the name of the " head " (Scolex), is provided with four 

 or two suckers, and usually with curved claw-like hooks. The dorsal 

 and ventral surfaces of the head are perfectly identical, so that the ar- 

 rangement of the hooks presents a strikingly, radiate appearance. By 

 means of this apparatus the worms fasten themselves on the intestinal 

 membrane of their hosts, which (except in the case of the otherwise peculiar 

 Archigetes) all belong to the vertebrates. The nurses develop in a more 

 or less complicated fashion as so-called " bladder-worms "from little round 

 six-hooked embryos. The latter inhabit very diverse but usually paren- 

 chymatous organs of the higher and lower animals, and are thence 

 passively transferred to the intestine of their subsequent host. 



What we have here described as a polymorphic colony is that 

 creature which is commonly called a " tape-worm," and regarded as a 

 simple individual with a head and jointed body. At first sight this 

 common conception seems to be perfectly justified, but it can hardly 

 be upheld in face of the facts of the case. It is, for instance, scarcely 

 consistent with our idea of an individual animal to observe that not 

 only can the terminal " ripe " joints of the tape-worm separate them- 

 selves with the greatest facility, and creep about for a while as inde- 



