220 



ANNELIDA. 



Fig. 107. 



the simplest way of obtaining a correct idea of the relative sizes and 

 general arrangement of these organs is to make a cast of their internal 

 cavities when in a state of distention ; this is 

 readily effected by placing a dead leech in 

 warm water until it is slightly heated : in this 

 state, the pipe of a small injecting syringe 

 can be introduced into the oesophagus, so as to 

 fill the stomach and caeca with common wax 

 injection ; and if the body be immediately 

 removed into a vessel of diluted muriatic 

 acid, the soft parts will be speedily destroyed, 

 leaving an exact model of the interior. It 

 will then be seen that the lateral caeca in- 

 crease gradually in size as they approximate 

 the posterior extremity of the body, until the 

 last pair (d) become so large as nearly to fill 

 up the space intervening between the end of 

 the stomach and the anal boundary of the 

 visceral cavity. 



(577.) The small size of the intestine (e) 

 when compared with the capacious stomach 

 described above is remarkable : it commences 

 by a minute orifice at the termination of 

 the digestive cavity, and becoming slightly 

 enlarged, passes in a straight line, lodged be- 

 tween the two posterior caeca, to the anus, 

 which is an almost imperceptible aperture 

 placed at the root of the posterior sucker : 

 four small and apparently glandular masses 

 are appended to this short canal ; but their 

 nature is unknown. The entire alimentary 

 apparatus is retained in situ by numerous 

 membranous septa (m m), passing between its 

 outer walls and the muscular parietes of the 

 body. 



(578.) In the Leech, the circulating system is more highly developed 

 than in any other Annelid*. The presence or absence of a heart-like 

 centre to this system in this class of animals is by no means the true 

 criterion of the degree of its evolution. The amount of blood relatively 

 to the size of the body, the degree of capillary subdivision which occurs 

 on the periphery of the blood-system, and the proportion of the latter 

 to the peritoneal fluid, form far more correct indications. In the Leech 

 there exists no free space between the intestine and the integument : to 



* Vide Dr. Williams' s Eeport on the British Annelida, in the Reports of the British 

 Association for the Advancement of Science for 1851. 



Digestive organs of the Leech 

 (Ilirudo mediclnalis) : b, mouth; 

 h, i, interior of the stomachal 

 cavity, exhibiting the openings 

 of the lateral caeca (fc) ; g, first 

 pair of stomachal caeca; d, last 

 pair, extending backwards on 

 each side of the intestine e. 



