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THE CLASSIFICATION OF ANIMALS 39 



point there is an enormous advance over the structure ex- 

 hibited by the Platyhelminthes ; the digestive tube has two 

 openings, one in front called the mouth, used for taking in food, 

 and one behind termed the anus, for rejecting undigested material 

 The Nemertinea are further "distinguished 

 by possessing a kind of protrusible trunk at 

 the front end of their bodies, which when 

 not in use is retracted into a sheath. 

 This trunk is termed the proboscis, and 

 it can be rapidly shot out and rolled 

 round the victims on which they prey; 

 for the Nemertinea are active predatory 

 animals. 



Another phylum whose members are 

 termed " worms " is that of the Nematoda 

 or Thread-worms. These animals are in 

 the great majority of cases parasites ; they 

 are practically ubiquitous, and are of enor- 

 mous economic and medical importance. 

 Like the Nemertinea, and indeed all the 

 other groups which we shall still have to 

 consider, they possess both mouth and anus. 

 Their leading feature is the extreme simpli- 

 city of their anatomy. The digestive tube 

 is simple and straight, without complica- 

 tions. The muscular system consists of a 

 single layer of longitudinal muscles ; outside ^ 12 __ A 

 these comes the skin, which consists of a 

 layer of cytoplasm with scattered nuclei, in 

 which separate cells cannot be distinguished. 

 This skin gives rise to the most character- 

 istic feature of the group, viz. a thick, smooth, 

 glistening cuticle, i.e. a sheath of dead 

 secretion which is produced by the transformation of the cyto- 

 plasm of the skin. This cuticle is very elastic, and, as the 

 young animal grows in length, it stretches ; but its elasticity 

 has limits it is eventually burst and cast off, and the animal 

 then grows rapidly in length, and forms a new cuticle by 

 renewed secretion. This process, which is termed moulting, 

 or ecdysis, occurs four or five times in the animal's life. 



mertine worm 

 viewed from 

 upper surface; 

 pr, proboscis ; 

 sh, sheath of 

 proboscis. 



