218 ANNELIDA. 



internal layer is made up of longitudinal muscles, extending from one 

 end of the creature towards the opposite. Such an arrangement is 

 evidently adequate to the production of all needful movements, and 

 capable of giving rise to all the motions connected with the elongation, 

 contraction, or lateral inflexions of the body used in progression. 



(571.) At each extremity of the animal, the muscular coat expands 

 into a flattened fleshy disk, composed of circular and radiating fasciculi, 

 which, when applied to a smooth surface, perform the office of suckers, 

 and thus become important instruments of prehension. There are no 

 vestiges of external limbs ; nevertheless, with the simple mechanism 

 above described, the Leech is able to crawl with considerable rapidity 

 along the surface of subaquatic plants, or even to swim with much 

 facility through the water. The first method of locomotion is accom- 

 plished by means of the terminal suckers. Supposing the posterior disk 

 to be attached, the animal elongates its body to the utmost, and then 

 fixes the sucker placed at the opposite extremity ; this done, the hinder 

 parts are drawn forward and again fixed, preparatory to a repetition of 

 the process. In swimming, the whole body is elongated, and by some 

 partial contractions of the muscular integument, not precisely under- 

 stood, assumes the appearance of a flattened band ; in this condition the 

 Leech makes its way through the element it inhabits by successive undu- 

 latory movements of the body, performed with much grace and elegance. 



(572.) The mouth of the Leech is an exceedingly complete apparatus, 

 adapted not only to the destruction of minute aquatic animals that con- 

 stitute its usual food, but, as is universally known, admirably fitted to 

 extract blood from the higher animals ; combining, in its operation, the 

 offices both of the cupping-glass and the scarificator. 



(573.) The mouth is situated near the centre of the anterior sucker, 

 so that the oral aperture is firmly applied to any surface upon which 

 this part of the animal is fixed. Around the entrance of the oesophagus 

 are disposed three minute cartilaginous teeth, imbedded in a strong 

 circle of muscular fibres (fig. 106, A). Each tooth has somewhat of a 

 semicircular form, and, when accurately examined with a microscope, 

 is found to have its free margin surmounted with minute denticulations 

 (fig. 106, B), so as to resemble a small semicircular saw. On watching 

 a leech attentively during the process of biting, the action of these 

 teeth is at once evident ; for, as the skin to which the sucker is ad- 

 herent is rendered quite tense, the sharp serrated edges of the teeth are 

 pressed firmly against it, and, a sawing movement being given to each 

 cartilaginous piece by the strong contractions of the muscular fibres 

 around the neck, these instruments soon pierce the cutis to a consider- 

 able depth and lay open the cutaneous vessels, whence the creature 

 sucks the fluid which its instinct prompts it to seek after with so much 

 voracity. The position of the teeth around the opening of the mouth, 

 as represented in the subjoined figure (fig. 106, A), will at once explain 



