1 50 LEECHES. PART m. 



of which, is connected with an intestinal canal, which 

 ends in a vent in the middle of the terminal sucker. 



Within the mouth there are three crescent-shaped 

 jaws, presenting their convex edges towards the cavity 

 of the mouth, beset with from seventy to eighty teeth, 

 formed of a highly refractive crystalline substance re- 

 sembling glass. The leech makes a vacuum with its 

 sucker, which forces the part to which it is applied into 

 contact with the three-toothed jaws, which are moved 

 sidewise by strong muscles, and saw through the skin 

 and small bloodvessels below it. 



The leech, like the other Annelids, has two distinct 

 systems of circulating liquids, one red, the other colour- 

 less. The red liquid or blood is kept in circulation by 

 the pulsations of a heart, or rather a contractile vessel 

 behind the head. It is carried away from the heart by 

 a pulsating canal passing along the back of the leech, 

 and is brought back to the heart by a similar canal ex- 

 tending along its ventral side. During this course, por- 

 tions of the liquid are sent off through veins to different 

 parts of the body. The respiratory organs of the leech 

 are pores arranged at regular distances on each side of 

 the body which open into little sacs having capillary 

 bloodvessels distributed under the skin through which 

 the blood is aerated. 



The colourless liquid which contains many organic 

 molecules, occupies the space between the alimentary 

 canal and the inner wall of the body, from whence it 

 passes into canals which ramify extensively, but are not 

 furnished with returning passages. This liquid forms a 

 support to the muscles of the skin, and is kept in circu- 

 lation by the motions of the leech. 



Fig. 131 shows the highly developed nervous system 

 of the leech. From the double lobe of the brain ten 

 optic nerves go to the bases of ten black eye-specks, 

 which mark at equal distances the upper margin of the 



