ANNELIDA. 375 



The head in most of these animals is distinctly marked, and furnished 

 with organs of sense, such as eyes, tentacles, and in some instances 

 auditory vesicles, containing otolithes. The nervous system, in the 

 higher forms, exhibits the articulate type of structure very distinctly ; 

 it usually consists of a series of ganglia running along the ventral por- 

 tion of the animal, and united by a pair of slender filaments, by which 

 they also communicate with the central mass, or bi-ain, which is en- 

 closed in the head. They are divided into two groups, characterised 

 by the presence or absence of external respiratory organs. The abran- 

 chiate Annelides include the Suctoria or LeecJies, and the Scoledna or 

 Earthworms. The branchiferous group are again subdivided into 

 two orders : the Tvhicola, animals having a tube for their habita- 

 tion, and the Errantia, those having no such protection. Of the ani- 

 mals belonging to the order Suctoria, the medicinal leech is a familiar 

 example. Their motions are eflfected by undulations of the body whilst 

 swimming, or by the alternate attachment of the sucking disks with 

 which the two extremities of their bodies are furnished. 



The medicinal leech puts forward strong claims to our attention, on 

 the ground of the services which it renders to mankind. The whole of 

 this 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 w'hich 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. 

 119). "This piece of mechanism," says Profes- 

 sor Eymer 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 within 

 the margin of this hole three beautiful little 

 semicircular saws are situated, arranged so that 

 their edges meet in the centre. It is by means 

 of these saws that the leech makes the incisions 

 whence blood is to be procured, an operation 

 which is performed in the following manner; No 

 sooner is the sucker firmly fixed to the skin, fig- US- 



than the mouth becomes slightly everted, and ^louth of Leech. 

 the edges of the saws are thus made to press upon the tense skin ; a 

 sawing movement being at the same time given to each, whereby it is 

 made gradually to pierce the surface, and cut its way to the small 



