Page:The Zoologist, 1st series, vol 1 (1843).djvu/48

20 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 striæ, and the primitive fibrillæ of which each fibre is composed are barely perceptible, nothing but a slight dotted appearance being presented, which seems to be characteristic of the muscular fibres in these Hirudinidæ; 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 œsophagus, which are here thrown into an orbicular figure: at the point where the mouth joins the œsophagus 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 extremities are slightly indented; the middle tooth of the fifteen is the largest, 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.

The jaws themselves are firmly imbedded in strong muscles, which ive disposed in such a manner as to move them backwards and for