208 



MANUAL OF ZOOLOGY. 



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

 of a spongy vascular tissue. The pseudo-haemal system con- 

 sists principally of four great longitudinal trunks, connected 

 by lateral vessels, and devoid of any special dilatations.* Re- 

 spiration appears to be partly effected by means of a number of 

 sacs, which are formed simply by an involution of the integu- 

 ment, and which open externally by minute apertures, termed 

 "stigmata." 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 inter- 

 nally, and only open externally by the 

 " stigmata." In some of the Hirudinea, 

 however, the "segmental organs" agree 

 with those of the great majority of the 

 Annelides in not only opening exter- 

 nally, but in also communicating inter- 

 nally with the perivisceral cavity. The 

 segmental organs of the Leeches differ 

 further from those of the other Annelida 

 in not being in any way connected with 

 the process of reproduction. 



The nervous system consists of a 

 prae-cesophageal ganglion, which gives 

 branches to a number of simple eyes, 

 or ocelli, which are placed on the head, 



Fig. 7 8.-Hirudine a . a The and which is united by lateral oesopha- 

 Medidnai Leech (San? uisn- g^al cords to the ventral gangliated 



ga.officina.lis), natural size; 

 Anterior extremity of the 



same magnified, showing the 



sucker and triradiate jaws ; ....... , T , 



c One i of the jaws detached, individual, but the Leeches are never- 

 semicircular theless incapable of self-fertilisation. 



The sexes are united in the same 



t s ^thed s mar|in. 



* If Branchiobdella be regarded as a true Leech, then the absence of 

 gills is not universal in this order, for it possesses branchiae. It has two 

 suckers, and is parasitic on the Torpedo and on the gills of the Cray-fish. 



