104 ZOOLOGY. 
variegatum and Milleria 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 2. 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 dendrocelous 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 ceca. Uni- 
cellular glands open into the pharynx. In one genus (Am- 
philina) there is no digestive canal. 
The Trematodes usually possess what the Turbellarians 
do not have, a sucking-disk (Fig. 68, B, s), 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. 
