20 Annelides. 



sucking disc, and forms a band about a line in breadth around this 

 part. The radiating fibres can very well be seen extending from the 

 centre to the margin of the anal disc. The muscular fibres themselves, 

 when examined microscopically, with a power of five hundred linear, 

 exhibit little or no trace of transverse striae, and the primitive fibrillae 

 of which each fibre is composed are barely perceptible, nothing but 

 a slight dotted appearance being presented, which seems to be cha- 

 racteristic of the muscular fibres in these Hirudinidae ; the average 

 diameter of the fibres was about the one thousandth of an inch, and 

 many of them appeared to be pointed at one end. 



Digestive system. The mouth is situated at the anterior extremity 

 of the body, on the ventral surface, immediately behind the sucking- 

 disc ; it is of an oval shape, and composed of fleshy lips, which are 

 rendered thick and soft by the concentration of the fibres investing 

 the whole of the body, and by those which are continuous with the 

 muscular oesophagus, which are here thrown into an orbicular figure : 

 at the point where the mouth joins the oesophagus there are three 

 cartilaginous jaws ; * they are placed in a radiating manner, and form 

 with each other an angle of about 180° ; they project some little way 

 into the mouth, and in the medicinal leech they can readily be felt to 

 grate against a metal instrument when it is passed into the mouth ; 

 each jaw is of a semicircular figure and of a white colour, and is 

 provided with fifteen teeth-like appendages (fig. e) ; they are of a 

 flattened or conical figure, and are arranged like so many inverted V's 

 on the upper curved surface of the semicircular cartilaginous jaws ; 

 they are broader at the base than at the apex, and the basal extremi- 

 ties are slightly indented ; the middle tooth of the fifteen is the lai*- 

 gest, and the others gradually diminish in size to the outside, where 

 they are only half as large as the middle one. In leeches which have 

 been dead some little time, and slight decomposition has taken place, 

 these conical teeth, by pressure, will readily divide at their apices, 

 and give one the idea of there being two rows instead of one, while, 

 in the recent state, the line of separation is scarcely perceptible, but 

 by macerating them they will easily separate : it must have been this 

 accidental separation that has led many authors, and amongst them 

 Moquin Tandon, to describe and figure two rows of teeth in each jaw. 



'Ihe jaws themselves are firmly imbedded in strong muscles, which 

 are disposed in such a manner as to move them backwards and for- 



* Wliat arc here termed jaws are called teeth hy most authors ; I apply the tena 

 teeth to the little calcareous hodies on their free surfaces. 



I 



