CHAPTER V. 



THE HIRTTDINEA, THE 0LIG0CH.ETA, THE POLYCH^TA, THE 

 GEPHYREA. 



The Hirudinea. — The Leeches are aquatic or terrestrial, 

 more or less distinctly segmented, vermiform animals, most 

 of which suck blood, though some devour their prey. The 

 ectoderm is a cellular layer, covered externally by a chitinous 

 cuticula, and, except in Malacobdella, devoid of cilia. Very 

 commonly it is marked by transverse constrictions into rings, 

 which are more numerous than the true somites as indicated 

 by the ganglia and the segmental organs ; and simple glands 

 may open upon its surface. One or more suckers, which 

 serve as organs of adhesion, are developed upon it. In some 

 (Acanthobdella) bundles of setae are present ; in others (Bran- 

 chellion) the sides of the body are produced into lobe-like 

 appendages ; but none have true limbs, unless the lateral ap- 

 pendages of Histriobdella are to be considered as such ; nor 

 are the anterior segments of the body so modified as to give 

 rise to a distinct head. 



The mouth is generally situated at the anterior end of the 

 body ; the anus at the opposite extremity, on the dorsal side 

 of the terminal sucker. The buccal cavity may be armed 

 with several serrated chitinous plates, as in the Medicinal 

 Leech, where there are three such teeth. By their aid the 

 Leech incises the skin and gives rise to the well-known tri- 

 ardiate mark of a leech-bite. The buccal cavity usually 

 opens into a muscular, sometimes protrusible, pharynx, from 

 which a narrow oesophagus leads into a stomach, which is fre- 

 quently produced into lateral caeca. In the Medicinal Leech 

 (Fig. 49), for example, there are eleven pairs of such caeca, 

 increasing in length and capacity from before backward. 

 From the stomach a narrow intestine leads to the anus. In 



