160 STRUCTURE AND CLASSIFICATION 



last, which constitutes the tail, is precisely like the 

 one above or below it, save in size ; those of the middle 

 portion of the body being frequently larger than the 

 rest. The head is frequently furnished with eyes, and 

 with more or less perfectly formed tentacula, or feelers. 

 It contains the mouth, which in many species is armed 

 with formidable jaws, or with cutting teeth, which fur- 

 nish these voracious creatures with a powerful means 

 of attacking their prey. Most, if not all of them, are 

 carnivorous. The blood-sucking propensities of some, 

 as of the common Leech, are proverbial. The blood of 

 all these worms is remarkable for its red colour, and it 

 circulates through a double system of arteries and veins. 

 The mode in which this blood is aerated varies con- 

 siderably in different members of the class ; and as the 

 differences of breathing apparatus indicate important 

 varities of habit among these animals, these differences 

 have been happily chosen by Cuvier, as the basis on 

 which his systematic division of ANNELIDES, or Red- 

 blooded Worms, is constructed. He divides this class 

 of animals into three groups or orders. In the first, 

 which he calls Abranchiata, there is no external breath- 

 ing apparatus ; but along the sides of the body are 

 disposed a number of minute holes, by which the sur- 

 rounding medium, be it air or aerated water, is freely 

 admitted into little bags, concealed beneath the skin. 

 Over the membranous surface of these bags the blood- 

 vessels form a delicate network, by which the contained 

 fluid is exposed, through the thin membranous wall, to 

 the action of the air or water. To this group belong 

 the Earth- Worms, the Leeches, and several creatures of 

 similar habits, frequenting muddy places in sestuaries 



