x PHYLUM ANNIILATA 505 



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 Macrobdella valdivania is said 

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

 a segment varies from two to fourteen and is constant in each 

 species, but the general form of the body is remarkably uniform, 

 the external differences between various species depending largely 

 on colour and on the development of papillae, which in some cases 

 are large and prominent. Setae are absent in all except one genus, 

 Acanthobdella, which has two pairs on each side of the first five 

 segments. 



The proboscis (Fig. 421), 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 Sipunculoidea. 



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. 421, gul.) leading from the 

 pharynx to the crop (cr.), and thrown into a coil when the proboscis 

 is retracted. Among the Gnathobdellida the median jaw is absent 

 in some land-leeches, and in other species all three jaws are rudi- 

 mentary or absent. 



Blood-system and Ccelome. In the medicinal leech, as 

 we have seen, there are lateral vessels with contractile muscular 

 walls and dorsal and ventral sinuses with non-contractile walls. 

 All of these seem to be formed, in this and the rest of the Gnatho- 

 bdellida, from ccelomic spaces, and none of them, therefore, correspond 

 developrnentally to the true blood-vessels of the Chsetopoda, which 

 are formed independently of the coelome. In the rest, however, 

 the dorsal and ventral vessels are developed in the manner of true 

 blood-vessels, while the lateral vessels or sinuses are always 

 ccelomic in origin. 



In Pontobdella, one of the Rhynchobdellida, there are dorsal 

 and ventral as well as lateral vessels, and lateral as well as dorsal 

 and ventral sinuses, and in each case the vessel is enclosed in the 

 corresponding sinus. The ventral sinus also contains the nerve- 

 cord and the ovaries, and offshoots of it surround the lestes and the 

 nephrostomes. In the Rhynchobdellids in general the ccelomic 

 spaces remain fairly extensive, and are lined by a ccelomic epithelium. 



