CCELOM AND VASCULAR SYSTEM IN THE LEECH. 489 



The annular channels of the dorsal region always derive 

 from the contractile system. Those of the ventral region 

 generally belong to the sinus system. The two sets inter- 

 digitate, an annular sinus passing upwards between two 

 annular vessels, and they only communicate by means of fine 

 capillaries of the epidermal plexus. There are a considerable 

 number of annular channels to each segment. 



It has already been mentioned that a dorsal annular vessel 

 may reach round to the ventral region ; this happens in the 

 vicinity of the nephridiopore, and is shown in fig. 14. In 

 such cases the annular vessel comes into communication with 

 capillaries from the latero-abdominal vessel, which then bear 

 just the same relation to it as the capillaries from the dorsal 

 sinus in the upper region. 



These ventral annular vessels of dorsal origin bend inwards 

 towards the ventral sinus, and pass upwards by the side of the 

 gut to the dorsal region, where they break up into capillaries 

 to be distributed to the botryoidal tissue and dorsal epidermal 

 plexus. They form, in fact, the vertical channels already men- 

 tioned. 



There remain to be described the relations of the botryoidal 

 tissue. The channels are lined within with the well-known 

 yellowish-brown cells, the outer region of which is filled with 

 coarse pigmented granules, and the inner deeply-staining half 

 formed of comparatively clear protoplasm.^ 



The botryoidal vessels lie chiefly in a dorsal and ventral 

 mass on each side, between the alimentary canal and the 

 muscles of the body-wall in the general parenchyma. On the 

 outer side the botryoidal vessels communicate with the inter- 

 mediate capillary plexus of sinus origin (figs. 7 and 8). Occa- 

 sionally in the ventral region (and perhaps elsewhere) they 

 open into capillaries of the contractile system (fig. 14). On 



' It is, apparently, this deeply staining region of the cells which has been 

 mistaken by Graf (3) for an inner coat in Nephelis. There can be no doubt 

 that in Hirudo, Aulostoma, and Nephelis there is no such inner lining, and that 

 the previous observers were quite correct in describing the brown cells as 

 bathed by the hsemolymph. 



