146 ZOOLOGY. 



variegatum and Mutter ia lecanora, two East Indian Holo- 

 thurians. 



The Planarian worms merit careful consideration, as it is 

 possible that the Mollusca have originated from primitive 

 forms resembling them. 



Order x!. Trematodes. Having studied the Planarians, 

 we shall be able to appreciate the characteristics of the Tre- 

 matode worms, which are all parasitic, and are constructed 

 on the dendrocoelous planarian type, more or less modified 

 by their parasitic life, some being external, but most of 

 them internal parasites. They closely agree with the Tur- 

 bellaria in form, never being segmented. The mouth-open- 

 ing is usually situated near the fore-end of the body (some- 

 times in the centre), leading by a muscular pharynx to the 

 digestive canal, which is forked and ends in two coeca. Uni- 

 cellular glands open into the pharynx. In one genus (Am- 

 pliilina) there is no digestive canal. 



The Trematodes usually possess what the Turbellarians 

 do not have, a sucking-disk (Fig. 98, B, $), situated a little 

 behind the middle of the body, by which, they adhere to the 

 walls of the organ of the host they inhabit. The so-called 

 water-vascular* or excretory system forms a network of 

 vessels branching from two main lateral tubes, which unite 

 to form a contractile vesicle ending in a terminal pore, or 

 the main branches may end in two or more lateral pores. 



The fact that there is no anal opening seems to confirm 

 the idea that the water-vascular system is excretory, thus 

 affording the only outlet for the waste products of diges- 

 tion. There are no blood-vessels or respiratory organs, and 

 the surface of the body is not ciliated-except in the embryo. 

 The nervous system is usually represented by a single gan- 

 glion, like that of the Turbellarians. Eye- spots are some- 

 times present in the young, which, with other points in their 

 organization, tends to show that the Trematodes have origi- 

 nated from Turbellaria, having been modified by their para- 



* That the so-called water- vascular system is mainly at least excretory 

 in its function seems proved by the fact that the fluid is watery and 

 contains granular concretions, thus resembling the urinary excretions 

 of the higher animals. 



