ANNULOSA : ANNELIDA. 163 



and devoid of any special dilatations. Respiration appears to 

 be effected by means of a number of pulmonary 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. 



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, 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. 



The common Horse-leech is not provided with any dental 

 apparatus; but the Medicinal Leech (Sanguisuga officinalLs) 

 (Jig. 45) 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. OLIGOCILETA (Terricola). The members of this 

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

 Water- worms (Naididce^), are distinguished by the fact that 

 their locomotive appendages are in the form of chitinous setae 

 or bristles, attached in rows to the sides and ventral surface 

 of the body. They are all hermaphrodite. 



In the common Earth-worm (Lumbricus) the body is cylin- 

 drical, attenuated at both extremities, and carrying in the 

 adult a thickened zone, which occupies from six to nine rings 

 in the anterior part of the body, is connected with reproduc- 

 tion, and is termed the ' clitellum,' or 'saddle' Locomo- 

 tion is effected by eight rows of short bristles or setae, four of 

 which are placed laterally and four on the ventral surface of 

 the body ; these representing the foot-tubercles of the higher 

 Annelides. The mouth is edentulous, and opens into a short 

 oesophagus, which leads to a muscular crop, or ' pro-ventri- 

 culus,' succeeded by a second muscular dilatation, or gizzard. 

 The intestine is continued straight to the anus, and is con- 

 stricted in its course by numerous transverse septa, springing 

 from the walls of the perivisceral cavity. The pseudo-haemal 



M 2 



