PLATYHELMIA 



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PHYLUM I. PLATYHELMIA Vogt, 1851. 



Synonyms. — Platodes Leuckart, 1854 ; Platyhelminthes Gegenbaur, 

 1859; Platyhelminthes Minot, 1877. 



Definition. — -Platyhelmia are bilaterally symmetrical Metazoa, 

 with dorso-ventrally flattened bodies, and without a true coelom. 



Morphology. — ^The Platyhelmia are flat-worms, with an oval or 

 tape-like body, which is either covered by a ciliated epithelium, as in 

 the free-living Turbellaria, or with a cuticle under which the surface 

 epithelium has sunk into the parenchyma, forming the so-called 

 subcuticular layer in the parasitic Trematoda and Cestoda. A well- 

 developed musculo-dermal layer is present, but there is no coelom. 



The mouth is generally situated at the anterior end, when present, 

 but it may be moved to the inferior surface, and opens, via a 

 pharynx, into a forked or branched gut, which has no anal aperture. 

 The alimentary canal may, however, be wanting (Cestoda). The 

 excretory system begins in the so-called flame cells — i.e., cells 

 provided with a leash of cilia from which fine channels run, uniting 

 together to form larger channels. These ultimately empty into a 

 pair of laterally placed canals, opening to the exterior separately 

 or together, often through an excretory vesicle. 



The nervous system consists of a large, bi-lobed, cerebral gan- 

 glion, with nerves running forwards and backwards. 



The Platyhelmia are mostly hermaphrodite, but may rarely be 

 unisexual; the ova are produced in the ovary, near which they are 

 fertilized, and then, after obtaining food-yolk from the yolk reservoir, 

 which has received it via the vitelline ducts from the yolk glands, 

 they acquire a shell in a shell gland, and then enter a uterus, through 

 which they slowly pass to the exterior. 



The male organs consist of testes, vasa deferentia, vesicula 

 seminalis, and a cirrus pouch, with a cirrus and a so-called prostate 

 gland. 



Classification. — -The Platyhelmia are classified into : — 



Class I. Turbellaria.— Yvee-living flat-worms, covered with cilia. 



Class II. Trematoda. — -Parasitic flat-worms with an alimentary 

 canal ; cuticle non-ciliate. 



Class III . Cestoidea.—Vd.YssHic flat- worms without an alimentary 

 canal; cuticle non-ciliate. 



Class L does not enter into the subject under discussion. 



CLASS IT TREMATODA Rudolphi, 1808. 

 Synonyms. — Sucking Worms. 



Definition. — -The Trematoda are parasitic Platyhelmia, which re- 

 tain the mouth and alimentary canal, but in which the epidennis 

 not merely loses its cilia during development, but is sunk into the 

 mesoblast after secreting the chitinous cuticle. Suckers are de- 

 veloped in the region of the mouth, and also on the ventral surface. 



Remarks. — ^The knowledge of the Trematoda began in 1379, when 



