8 



ENTOZOA. 



tions adopted by Oersted of Oopenliagen may be regarded as the 

 most satisfactory. 



In common with the Trematoda, the Turbellarians have their 

 bodies composed of soft parenchymatous tissue, and in this loose 

 substance the various specialized organs are lodged without the 

 intervention of any perivisceral cavity. Some of the animals have 

 a flattened form, others are cylindrical, whilst a third kind are 

 remarkably attenuated, and more or less barred by transverse 

 rug^, which form, as it were, a series of spurious joints or articu- 

 lations. The mouth and digestive apparatus are well developed, 

 but there is no certain evidence as to the existence of an anus in 

 any of the species. 



PlanaridcB. — The resemblance of the individual members of 

 this family to the common liver fluke is very striking, not only in 

 the matter of external form and aspect but also as regards the 

 conformation of the intestinal system, which is essentially dendritic 

 or multiramose. In the group termed Dendrocoehans by Oersted, 

 we have a smooth, flat, inarticulate body, and a capacious mouth, 

 which is situated on the ventral aspect, leading to a large stomach, 

 the latter giving ofi" a series of regularly branching coeca. The 

 nervous system is well developed, the two cerebral gangha trans- 

 mitting filaments to the eyes or ocelli, which in some of the species 

 are very numerous [Planaria, Eolidoceros) , whilst in others there 

 are but two (Mesostoma) . The male and female reproductive organs 

 are conspicuous, as obtains in the hermaphroditic trematodes, the 

 outlets of these organs being generally separate and distinct. A 

 water-vascular system has been described by several observers 

 (Siebold, Ehrenberg), but its existence has been either doubted 

 or altogether denied by others. 



The power of reproduction in the Planarians is very striking, 

 but their mode of development is as yet only very partially under- 

 stood. Beside the ordinary mode of sexual increase, some species 

 propagate by natural fission, and in this latter method almost any 

 fragment of the body seems capable, on being cast off", of realizing 



