1 82 MANUAL OF ZOOLOGY. 



sometimes edentulous, but is usually armed with teeth. The 

 alimentary canal is short, and is united to the skin by means of 

 a spongy vascular tissue. The pseudo-haemal system consists 

 principally of four great longitudinal trunks, connected by late- 

 ral vessels, and devoid of any special dilatations. Respiration 

 appears to be partly effected by means of a number of sacs, 

 which are formed simply by an involution of the integument, 

 and which open externally by minute apertures, termed " stig- 

 mata." In the common Leech there are about seventeen of 

 these vesicles on each side of the body, their openings being 

 placed on the abdominal surface. These saccular involutions 

 of the integument certainly secrete the mucus with which the 

 body of the animal is lubricated, and it is believed by some 

 that their function is solely excretory, and that they answer to 

 the kidneys of higher animals. In this case respiration must 

 be effected by the general surface of the body ; but there is 

 no reason why the same organs should not perform both func- 

 tions, since a close relationship subsists between the two. 

 These sacculi are generally known as the " segmental organs," 

 and in most of the Hirudinea they are closed internally, and 

 only open externally by the " stigmata." In some of the .Hiru- 

 dinea, however, the " segmental organs" agree with those of the 

 great majority of the Annelides in not only opening externally, 

 but in also communicating internally with the perivisceral 

 cavity. The segmental organs of the Leeches differ further 

 from those of the other Annelida in not being in any way con- 

 nected with the process of reproduction. 



The nervous system consists of a prae-oesophageal ganglion, 

 which gives branches to a number of simple eyes, or ocelli, 

 which are placed on the head, and which is united by lateral 

 cesophageal cords to the ventral gangliated chain. 



The sexes are united in the same individual, but the Leeches 

 are nevertheless incapable of self-fertilisation. Reproduction, 

 also, is always effected by means of the sexes, and never by 

 fission or gemmation. 



The common Horse-leech is not provided with any dental 

 apparatus; but the Medicinal Leech (Sanguisuga qfficinalis, 

 fig. 55) has its mouth' furnished with three crescentic jaws, 

 the convex surfaces of which are serrated with minute teeth. 

 This species is chiefly imported from Hungary, Bohemia, and 

 Russia. 



ORDER II. OLIGOCH^ETA (Terricola). The members of this 

 order, comprising the Earth-worms (Lumbricidcz) and the 

 Water-worms (Na'ididcz], are distinguished by the fact that their 

 locomotive appendages are in the form of chitinous setae or 



