188 LOWER INVERTEBRATES. 



which, being concealed in the body of their host, require little more than the appara- 

 tus for eating the victims that are also their dwellings, where they are well protected 

 from external attack. Parasites are always degraded in structure. Within the 

 present class we find a series of forms, beginning with those leading a free life and 

 ending with those which are parasitic during their whole existence ; with also inter- 

 mediate links between the extremes, species that are parasitic through a longer or 

 shorter term. In this series we find a progi-essive degradation, as the parasitism 

 increases until it becomes the complete master of the worm's whole life. The free • 

 living forms are the unaltered types, the undegenerated patterns of the class, of which 

 the parasites are the marred copies. Let us begin with the type, the planarians, or 

 so-called Turbellaria. 



Sub-Class I. — Tuebellahia. 



Oedek L — DENDROCCELA. 



If we scoop up some mud and plants from the bottom of a ditch in which the water 

 is tolerably clear, and let the collected mire settle in a basin of water, many strange 

 and interesting animals will be discovered, — many insect larvae, molluscs, Crustacea, 

 and worms. Among them, one usually discerns some short, very dark creatures, long 

 in shape, quite broad and thin, M'hich crawl about slowly, but almost incessantly, over 

 the sides of the basin and the various objects in it, or indeed sometimes along the 

 upper surface of the water itself ; their soft, flexible bodies are highly contractile, so 

 that the animal has hardly any definite shape, except indeed when crawling straight 

 forward ; for while thus progressing it always assumes a constant and characteristic 

 form, as shown. These worms belong to the genus Planaria, and are typical turbel- 

 larians. Of the genus several species have been desci-ibed, but it is diflicult to deter- 

 mine the specific characteristics of a Planaria. The best known and most common 

 form is the P. torva, found in Europe as well as America. The planarians do not 

 have any means of locomotion visible to the naked eye, yet a close examination will 

 disclose the existence of their motor organs, for it is possible to distinguish the pass- 

 ing and whirling of suspended particles in the water. These whirls are most charac- 

 teristic ; the very name of Turbellaria refers to them ; they are produced by immense 

 numbers of vibratile hairs or cilia which cover the body, especially on the ventral 

 side. Even in proportion to the small size of the worm the cilia are tiny, 

 but the united propulsions of the multitude of cilia are sufficient to achieve 

 the worm's locomotion. The planarians, after the excitement produced 

 by their transference has ended, subside into the inanimate sediment, amidst 

 which they are well concealed by their dark brown color. At night time 



they are more active, for it is then that they gratify their carnivorous 

 Fig. 164. — -^ . , , .•'.*•' 



Planaria voracity, but they seem to me very unmteresting, except from an anar 



tomical standpoint. 



In the same basin we may find, beside the black planarians, many other allied 



worms, the largest among which will probably be the whitish Dendrocoelum lacteum. 



Some individuals measure over three-quarters of an inch in length. Its natural 



habitat is on the under- side of stones and leaves. It is white, with a shimmer of gray, 



and so translucent that the digestive canal shines through, and as it is usually gorged 



with dark-colored food it appears very distinctly. It is not a simple but a branched 



