PARASITISM IN RELATION TO BIRDS. 43 



It is thus seen that the Cestoda illustrate enteric parasitism 

 with entire loss of alimentary canal. They usually alternate 

 between two hosts, and show a cestoid and cystic stage, such as, 

 for instance, Tcenia ccenurus, alternating between Dog (cestoid) 

 and Sheep (cystic). 



A general survey of Cestoda furnishes the following 

 characters : — 



(1) Elongated flat worms with their cuticle and generally 



segmented body. 



(2) Head may have hooks and suckers present. 



(3) No mouth or alimentary canal ; food being absorbed 



osmotically over the whole body-surface. 



(4) Life-history of two phases :— 



(i) The cystic, 

 (ii) The cestoid. 



(5) No sense-organs. 



(6) One or two longitudinally excretory vessels. 



(7) Keproductive organs discontinuous from one proglottis 



to the other. 



(8) Endo-parasitic. 



Each joint, or proglottis, contains at first male genitalia ; 

 later on the female organs are developed, and ultimately self- 

 fertilization takes place. Thus it is seen that the proglottides 

 near the head, or scolex, contain testes. Next come the ones 

 bearing testes and ovaries, and lastly appear segments bearing 

 only fertilized ova, and thus bearing embryos. 



A controversy has here arisen as to whether the Tapeworm 

 is to be considered as a colony, and that each segment is a 

 complete animal, or whether the whole number of segments is to 

 be taken as a single animal. Steenstrup, and, following his sug- 

 gestions, Leuckart, Von Siebold, Van Beneden, and others, came 

 to the conclusion that a jointed Tapeworm is really a colony 

 composed of two generations : — 



(1) The head and neck being derived from the larva. 



(2) The segments from the intercalated growth of the neck 



region thus constituting a second generation. 

 This colonial view was held from 1851-1880. Kecent work has, 

 however, shown that the Cestode, or Tapeworm, is more probably 

 a monozoon. 



e 2 



