HIRUDINIDiE. 573 



so as to admit insertion into decaying vegetables, and 

 when stretched to the utmost the root becomes an apex of 

 the slenderest cone." 1 



Planarioe multiply by eggs, and by spontaneous figura- 

 tion, in a transverse direction, each segment becoming a 

 perfect animal. Professor Agassiz believes that the infu- 

 sorial animals, Parametrium and Kolpoda, are nothing else 

 than Planarian larvae. 



Hirudinidce, the Leech tribe, are usually believed to 

 form a link between the Annelida on the one hand, and 

 the Trematoda on the other; but their affinities are closer 

 connected with the latter than the former. Totally deprived 

 of the characteristic setse of the Annelida, and exhibiting 

 no sectional divisions, they are provided with suckers so 

 constantly possessed by the Trematoda, and present no 

 small resemblance to them in their reproductive organs. 

 On the other hand, in the arrangement of their nervous 

 system aud in their vascular system, the HirudinidcB 

 resemble the Annelida. The head in most of these animals 

 is distinctly marked, and furnished with eyes, tentacles, 

 mouth, and teeth, and in some instances with auditory 

 vesicles, containing otolithes. The nervous system con- 

 sists of a series of ganglia running along the ventral 

 portion of the animal, and communicating with a central 

 mass of brain. 



The medicinal leech puts forth strong claims to our 

 attention, on the ground of the services which it renders 

 to mankind. The whole of the family live by sucking the 

 blood of other animals ; and, for this purpose, the mouth 

 of the leech is furnished with an apparatus of horny teeth, 

 by which they bite through the skin. In the common 

 leech, three of these teeth exist, arranged in a triangular, 

 or rather triradiate form, a structure which accounts for 

 the peculiar appearance of leech-bites in the human skin. 

 The most interesting part of the anatomy of the leech to 

 microscopists is the structure of the mouth (fig. 257). 

 "This piece of mechanism," says Professor Rymer Jones, 

 " is a dilatable orifice, which would seem at first sight to 

 be but a simple hole. It is not so ; for we find that just 



(1) Sir John Dalyell's Observations on some interesting Phenomena exhibited 

 \y several Species of Planar ice. 1814. 



