518 



ZOOLOGY 



various species depending largely on colour and on the develop- 

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

 Setae are absent in all except one gen'us, AcantkoMdla, which has 

 two pairs on each side of the first five segments. 



The proboscis (Fig. 414), 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, cor- 

 resjionding to the 

 eleventh pair in 

 Hirudo, while Ne- 

 phelis has none at 

 all. In the Rhyn- 

 chobdellida there is 

 a distinct slender 

 gullet (Fig. 4145'^s/.) 

 leading from the 

 pharynx to the crojD 

 (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 rudimentary or absent. 



The varying development of the blood-vessels and sinuses 

 presents many points of intere.st tending to explain the condition 

 of things described above in the medicinal leech. In the latter, 

 as we have seen, there are lateral vessels with contractile muscular 

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

 In Pontobclclla, one of the Rhynchobdellida, there are dorsal 

 (Fig. 415 '3, d.v.) and ventral (v.v.) as well as lateral vessels, and 

 lateral (^.s.) as well as dorsal and ventral sinuses, and in each case 

 the vessel is enclosed in the corresponding sinus. The ventral 

 sinus iy.s.) also contains the nerve-cord (n.c.) and the ovaries (pv.), 

 and offshoots of it surround the testes {ts.) and the nephrostomes 

 {nst.). This arrangement clearly suggests the partial obliteration, 



Fia. 414.— Proboscis of Clepsine. A, retracted; B, everted 

 ci\ crop ; gvl. gullet ; udh, mouill ; 'pr, Introvert ; s. gl, 

 s.ali\'ary glands. (After Bourne.) 



