V] CESTODA 111 



lucky chances as that of transference from one particular species of 

 animal to another animal of a definite kind must develop large 

 powers of reproduction. In the Trematode this is chiefly manifested 

 in the power of the larva to reproduce itself asexually, but in the 

 Cestoda the power is in most cases only developed when the animal 

 is in the adult condition. Considerable discussion has taken place 

 as to whether the process of strobilisation is, or is not to be 

 regarded as a production of new individuals. When we recollect 

 that the separated proglottides often retain life for some time after 

 being cast out of their host, it would seem that there was much to 

 be said in favour of regarding them as sexual individuals and the 

 head as an asexual one. The most probable view on the whole is 

 that of Lang, who suggests that in the ancestor of modern Cestoda 

 the hinder part of the body which contained the genital organs was 

 separated at maturity, as occurs in the case of some Polychaeta. 

 When the Cestode took to living in the alimentary canals of 

 Vertebrata, the abundant food supply and favourable temperature 

 stimulated the powers of regeneration so that the missing part was 

 quickly reproduced, and by the hurrying on of this process of 

 regeneration the process of strobilisation was evolved, exactly as 

 occurred in the Scyphistoma of Aurelia. It is interesting to 

 notice that Archigetes, found mature in the coelom of the Oligo- 

 chaet worm Tubifex, is the only Cestode which completes its 

 development elsewhere than in a Vertebrate, and in this case only 

 one proglottis is produced, which never separates from the head. 

 To the posterior end of this proglottis an appendage is attached 

 representing the bladder of the Cysticercoid, in which the six hooks 

 of the onchosphere still remain embedded. 



The Platyhelmintb.es are classified as follows : 



ClaSS L TURBELLARIA. 



Platyhelminthes with soft, usually leaf-like bodies and a ciliated 

 ectoderm : rhabdites are often present : free-living. 



Order 1. Rhabdocoelida. 



Turbellaria with a straight, rod-like alimentary canal and 

 a protrusible pharynx 



Sub-order 1. Acoela. 



The digestive system is represented by a mass of endo- 

 derm cells which contains no cavity : a short pharynx and 

 single otocyst are present. 



