SEA-FISHERIES LABORATORY. 418 
the digestive gland, lying immediately under the 
epithelium, passes ventrally and slightly forwards to the 
anterior end of the renal organ, where it enters the dorsal 
sinus. Branches join it from the digestive gland and 
stomach. 
A sinus-like plexus of vessels between the muscle and 
the visceral mass passes blood from the gonads and 
intestine to the sinuses communicating with the renal 
organs. The blood from the adductor passes along ill- 
defined paths in that muscle to enter these sinuses, thus 
completing the circuit. 
The greater part of the blood from the visceral mass 
and alimentary canal passes by very conspicuous veins on 
the surface of the gonads (fig. 18, |”. v.) to the sides of the 
renal organ, where they communicate with the numerous 
small vessels of the latter. 
Thus all the blood is brought to the renal organs, with 
the exception of that which proceeds by the pallial arteries 
to the mantle. ‘his will be considered later. 
The blood returning to the heart leaves the renal 
organ by a series of fine vessels in the outer wall which 
open into a wide passage, the entrance to the afferent 
branchial vessel (fig. 16, Br. aff.). This soon contracts in 
size, and the vessel runs along the ctenidial axis proximal 
to the accompanying efferent vessel. It communicates 
with a vessel or cavity on each of the respiratory 
expansions of the principal filaments, but no connections 
with any of the other gill filaments can be seen. 
The blood is brought from the gills, after aeration, 
to the heart, by means of the efferent branchial vessels, 
which, coming from the ctenidial axes, pass between the 
digestive gland and the adductor, and open into the 
narrow ends of the auricles, after receiving, at about the 
level of the dorsal extremity of the glandular part cf the 
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