78 OKHAMANDAL MARINE ZOOLOGY REPORT 



branchial circulation and passes to the gills for purification before being returned 

 to the heart, 



Pallial Sinuses. — Fig. 19 on Plate III. depicts graphically the arrangement and 

 course of the principal pallial sinuses within the left mantle. The principal one is a 

 great curved vessel, the anterior ventral pallial sinus (Sn.pall.a.v.), embracing within 

 its concavity the adductor muscle. It runs almost midway between the adductor and 

 the pallial edge, but rather closer to the former. The main branches feeding it appear 

 all to arise on the distal side, so their function is the draining of the pallial tissues 

 between the main sinus and the pallial edge. The sinus itself drains into the left 

 common efferent branchial trunk almost at its origin — that is, close to the fleshy 

 accessory excretory organ — but, just before doing so, receives a very large, 

 well-branched sinus — the anterior dorsal pallial sinus. 



A somewhat similarly disposed pair of sinuses {Sn.pall.p.v. and Sn.pall.p.d.) 

 drain the posterior region of the left pallial lobe, but the two members are smaller, 

 with fewer branches than the members of the anterior pair. They join just behind 

 the level of the lower margin of the heart and run forwards at this level as the 

 common posterior pallial sinus to empty into the left common efferent branchial trunk 

 at the same place as the anterior pallial sinal opening. In the right mantle the anterior 

 ventral pallial sinus is about as well developed as its counterpart on the left side ; 

 the dorsal member of the pair is weakly developed, and there appear to be no 

 equivalents present of the posterior pallial pair. The suppression of these latter and 

 the degeneration of the dorsal member of the anterior pair is correlated with the fact 

 that the upper part of the right mantle, instead of being free, as is that on the left, is 

 adherent to the surface of the visceral mass. As a consequence, the function of the 

 suppressed pallial sinus is usurped by the visceral sinuses of the right side, which, it 

 is noteworthy to remark, are extensively ramified and much more highly developed 

 than those on the left side of the visceral mass. In this connection it may be 

 remarked that, so far as the blood system is concerned, the left aspect is pre-eminently 

 the arterial, the right is distinctly the venous. 



Another point of great interest connected with the pallial sinuses is the association 

 of black pigment with their trunks and main branches. The intensity of pigmentation 

 varies considerably ; .sometimes the whole mantle is more or less deeply suffused 

 externally, the sinuses showing up as darker lines on a dusky ground, the pigment 

 being denser along their course ; in other individuals pigment is restricted to the 

 trunk vessels and their branches. The significance of this will be treated of elsewhere. 



Visceral Venous System. — The principal vessels of this group may be enumerated 

 as (a) the unpaired rectal sinus, {h) the paired main visceral sinuses, (c) the small 

 median sinus, and the paired renal sinuses. 



The first of these, the rectal sinus {Sn.r., fig. 15), runs along that posterior 

 portion of the kidney which overlies the rectum, and as they are closely associated, it 



