2 COMPARATIVE ANATOMY 



the gut is commonly branched so that its ramifications bring 

 the products of digestion to all parts of the body. 



The Platyhelmia are divided into three classes, the Turbel- 

 laria or flat -worms, the Trematoda or flukes, and the 

 Cestoda or tape-worms. The Turbellaria are delicate, soft- 

 bodied, flattened worms, shaped something like a leaf. Some 

 are marine, some terrestrial, and some live in fresh water. 

 Beudroccelum lacteum, a greyish- white species, is very common 

 in pools and ditches in many parts of England, and the marine 

 forms, which are often beautifully coloured with bands and 

 stripes, are common in rock pools between tide marks. 

 The Cestoda or tape-worms are generally elongated, and 

 composed of numerous segments or proglottides attached 

 to an anterior head armed with suckers and hooks. But 

 some forms, such as Archigates and Caryophyllaus are un- 

 segmented, and show no external distinction between head 

 and body. With one exception (Archigcetes), the sexually 

 mature Cestoda are parasitic in the alimentary canal of verte- 

 brated animals. The Trematoda are parasitic animals, usually 

 of flattened, leaf-like form, which in shape and organisation 

 closely resemble the Turbellaria. 



One of the best known of the Trematodes is the liver-fluke, 

 Distomum hepaticum, which in the sexually mature state 

 inhabits the bile-ducts of the sheep and other herbivorous 

 animals, and gives rise to the disease known as the liver-rot. 

 It is occasionally found in man. The mature liver-fluke is 

 flattened and leaf-shaped, with a blunt triangular projection 

 at its broader end. Numbers of them may be obtained by 

 slitting open the bile-ducts of the liver of an infected sheep. 



The mouth is situated at the extreme anterior end of the 

 animal, on the tip of the triangular projection mentioned 

 above. It is an oval aperture, lying in the middle of a cup- 

 shaped muscular organ known as the anterior sucker. The 

 posterior or ventral sucker is situated in the mid-ventral line, 

 just behind the junction of the triangular anterior projection 

 with the rest of the body. It has the form of a cup with 

 thick muscular walls, and serves only as a means of attach- 

 ment, having no aperture leading into the interior of the body. 



Nearly mid-way between the mouth and posterior of the 

 sucker, but nearer to the posterior sucker than to the mouth, is 



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