sect, ix PHYLUM ANNULATA 207 



dilation, and its form varies greatly according to whether it 

 is empty or gorged with blood. Posteriorly the crop com- 

 municates by a minute aperture with the stomach {st), a 

 tubular chamber which is the digestive portion of the canal ; 

 the blood is passed into it from the crop with extreme 

 slowness, and undergoes an immediate change, its colour 

 turning from red to green. The digestion of a whole 

 cropful of blood takes many months. The stomach is 

 continued into a narrow intestine (int) ; this passes into a 

 somewhat dilated rectum {ret), which turns slightly upwards 

 and opens by the anus {an) in the last annulus. 



The excretory system consists of seventeen pairs of 

 nephridia {nph. i-ij), situated in segments 6-22. A 

 typical nephridium (Fig. 120) has the general form of a loop 

 passing upwards from the ventral body-wall, produced into an 

 offshoot which extends inwards (mesially) to the correspond- 

 ing testis, and connected posteriorly with a small bladder or 

 vesicle (Fig. 120, vs). The free end is swollen into a lobed 

 mass which lies in a blood sinus (Fig. 1 14, nst) ; comparison 

 with other Hirudinea shows that this dilated end of the 

 nephridium represents a nephrostome which has lost its open 

 funnel-like end in correlation with the absence of a distinct 

 coelom. 



There is a complex vascular system, containing, like that of 

 the earthworm, red blood, the plasma coloured with hsemo- 

 goblin and containing sparsely distributed colourless corpus- 

 cles. But a striking difference from the preceding anneli- 

 dan types is found in the fact that the blood-containing spaces 

 are of two kinds, — blood-vessels proper, having muscular walls, 

 and blood-sinuses, the walls of which are devoid of muscle. 



The two principal blood-vessels are lateral in position 

 (Figs. 119 and 122, /. v), running fore and aft at the level of 

 the middle of the nephridia and uniting with one another at 



