47G ZOOLOGY SECT. 



This order includes Chpsine, parasitic on Snails, Frogs, &c. : 

 Piscicola, on fresh- water Fishes ; Pontobdella, and Branekellion, on 

 marine Fishes (Fig. 379). 



ORDER 2. GNATHOBDELLIDA. 



Hirudinea in which there is no proboscis : the mouth is usually 

 provided with three toothed jaws. 



This order includes Hirudo, the common Leech, parasitic on Ver- 

 tebrata ; Aulostoma, the Horse-leech, free-living and carnivorous ; 

 Trocketa, of subterranean habits ; Hccmadipsa, the Land-Leech. 



Systematic Position of the Example. 



Hirudo belongs to the family Hirudinidcc, of the order Gnatho- 

 Mellida. 



The absence of a proboscis places it in the order Gnat JwM ell ida : 

 the possession of ten eyes, and the presence of five rings to all the 

 segments except a few at the anterior and posterior ends, dis- 

 tinguishes it as a member of the family Hirudinidse : the genus 

 Hirudo is distinguished by the constant presence of twenty-six 

 segments and of 102 annuli. 



3. GENERAL ORGANISATION. 



In the essential features of their organisation the Leeches are a 

 very uniform group : there are, however, a few interesting modifi- 

 cations of structure which must be referred to. 



Form and Size. Most kinds do not exceed a few centimetres 

 in length, but the American species MacroMclla raldimana is said 

 to attain a length of 76 cm. (2-g- feet). The number of annuli to 

 a segment varies from three to five, but the general form of the 

 body is remarkably uniform, the external differences between 

 various species depending largely on colour and on the develop- 

 ment of papillse, which in some cases are large and prominent. 



The proboscis (Fig. 380), the possession of which is distinctive 

 of the Rhynchobdellida, is simply the retractile anterior end of the 

 body, which, by the action of special muscles, can be drawn back 

 into a temporary sheath. The organ is thus an introvert, like that 

 of Gephyrea Inermia. 



The chief differences in the structure of the enteric canal 

 depend upon the varying number, or, in some cases, the total 

 absence, of lateral pouches to the crop ; for instance, the horse- 

 leech has only a single pair, corresponding to the eleventh pair in 

 Hirudo, while Nephelis has none at all. In the Rhynchobdellida 

 there is a distinct slender gullet (Fig. 380, gul) leading from the 

 pharynx to the crop (cr.~), and thrown into a coil when the pro- 

 boscis is retracted. Among the Gnathobdellida the median jaw 



