I50 



PL A TYHELMINTHES. 



or " Sturdy " by pressure on the brain. Another small tape- 

 worm of the dogs, Tcenia echinococci^ has enormous cysts, 

 with secondary cysts and many "heads " {Echinococci), which 

 may occur in man, sheep or pigs. 



The members of the third class, or Turbellaria^ are 

 not parasites, but are terrestrial, marine, or freshwater. 

 In a number of characters they resemble the other classes. 

 The body is usually flattened dorso-ventrally. There is 

 no anus. There are neither vascular nor respiratory 

 organs. The excretory organs are of the "flame-cell" 

 type, there is a brain with two lateral nerves, and the 

 sexual organs are hermaphrodite and complex. On the 

 other hand, the Turbellaria usually have simple sense- 

 organs and the body is usually ciliated. 



In the Platyhelminthes we see a distinct advance in 

 structure when compared with the Cceleiiterata, The axial 

 or, at most, bi-plano-symmetry of the latter has given way 

 to piano-symmetry. The mesogloea of the Coelenterata, 

 with or without a few scattered cells, has given place to a 

 definite mesoderm formed into muscles, gonads and 

 parenchyma, and drained by a definite excretory system. 



PHYLUM PLATYHELMINTHES. 



Three-layered metazoa with bilateral symmetry (piano-symmetry) and flat- 



^ tened body. 

 Alimentary canal, when present, has no anus. 

 Mesoderm with no definite cavity and fills the space between skin and 



alimentary canal (parenchyma). 

 Excretory organs consisting of ducts opening to exterior and blind branches 



containing " flame-cells." 

 Nervous system usually with two lateral cords and an anterior paired brain. 

 No vascular nor respiratory systems. 

 Mostly hermaphrodite. 



Class I. 

 Cestoda. 

 Type — Tcenia. 

 Elongated flat-worms 

 with thin cuticle. 

 Hooks and suckers 

 on head. 

 No mouth nor alimen- 

 tary canal. 

 Life-history of two 

 phases — the cestoid 

 and the cystic. 

 No sense-orsr afi^.^^ 

 Endoparasitic. 



Class II. 



Trematoda. 



Type — Dzstomnm. 



1. Oval flat body with 



cuticle bearing fine 

 hooks and suckers. 



2. Anterior mouth, race- 



mose intestine. 



3. Life-history often of 



two phases, 



4. No sense-organs. 



5. Endoparasitic or ecto- 



parasitic. 



Class III. 

 Turbellaria. 

 Type — Mesostoma. 

 Oval flat body with 

 cilia and rhabdites. 



2. Ventral mouth, simple 



or branched intestine. 



3. Life-history simple. 



4. Paired eyes. 



5. Free — aquatic and ter- 



restrial. 



