188 LOWER I NVERTE URATES. 



wliich, being concealed in tlie l)ody of their host, require little more than the appara- 

 tus for eating the victims tliat arc also their dwellings, where they are well jjrotecte<l 

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

 present class we find .1 series of forms, beginning with those leading a free life and 

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

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

 shorter term. In this series we fiiul a progressive degradation, as the parasitism 

 increases until it becomes the comi)letc master of the worm's whole life. The free 

 living forms are tin' unaltered ty]ies, the undegenerated patterns of the class, of which 

 the ])arasites are the marred ccipics. Let us begin with the type, the planarians, or 

 so-called Turbellaria. 



Sub-Class I. — Tukbellaria. 

 Okdki: I. — DENDHOCa:LA. 



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 aninuds will be discovered, — many insect larva', molluscs, Crustacea, 

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

 in shape, (piite broad and thin, which crawl about slowly, but almost incessantly, over 

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

 npper 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 j)rogressing it always assumes a constant and characteristic 

 form, as show n. These worms belong to the genus Pkmaria, and are typical turbel- 

 larians. Of the genus several species have been described, but it is difficult to deter- 

 mine the specific characteristics of a PlaiMriu. 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 jjass- 

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

 teristic ; the very name of Turbellai-ia refers to them ; they are produced by immense 

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

 side. E\cu in prop<irtion to the small size of the w-orm the cilia are tiny, 

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

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

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

 wliicli they arc well concealed by their dark brown color. At night time 

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

 ' "I'laMirio voracity, but they seem to me \ ( vy uninteresting, excejit from an ana- 

 tomical standpoint. 

 In the s;nnc b:isin we may tiiul, beside the bl.-u-k ]ilanarians, many other allied 

 worms, the largest among which will probably be the whitish JJemlrocwlum luctetmi. 

 Some individuals measui'c over three-cpiarters of an inch in length. Its natural 

 habitat is on the underside 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 a])])ears very distinctly. It is not a simple but a branclied 



