80 i >K II A MANUAL MAKINE ZOOLOGY REPORT 



direct from the mantle lobes 1 and hinder portion of the body by way of the rectal and 

 the anterior pallial sinuses. 



Ki'tun tin' ventricle this mixture of pure and impure blood is pumped into all 

 parts of the visceral mass, muscles, and mantle. Thence it passes into irregular lacunar 

 spaces, from which the sinuses collect it. That collected by the pallial and rectal 

 sinuses goes, as already described, directly back to the heart, while that gathered into 

 the visceral sinal vessels is carried to the kidneys and the gills, whence, after 

 purification, it passes by the common efferent branchial trunks into the auricles. 



Reviewing the circulatory system of Placuna as a whole, several most noticeable 

 features become apparent. The more striking are, (a) the primitive tubular character 

 of the auricles in contrast with the specialised and centralised condition seen in the 

 more highly-developed pearl oyster, where the ventricle and auricles form a closely 

 associated or single mass ; (6) the marked asymmetry of the auricles and their 

 inter-communication by way of a particularly wide chaunnel relatively remote from 

 the ventricle ; (c) the presence of a single aorta, the anterior, consecpient upon the 

 suppression of the posterior, whereof the place is taken by a large branch given off by 

 the anterior aorta immediately after it leaves the ventricle ; following on this 

 suppression of the posterior aorta, we see that both the anterior and the posterior pair 

 of pallial arteries arise from the dorsal branches of the anterior aorta in the pearl 

 oyster the anterior and the posterior aorta supply respectively one pair of these pallial 

 vessels ; (<7) lastly, the absence of a pericardium surrounding the heart is particularly 

 striking, a condition of affairs confined among Lamellibranchs solely to the Anomiidese ; 

 this results in the ventricle lying free and uncovered within the mantle cavity. 

 Comparing with Anomia, we note in that genus that there are the vestiges of a 

 coelomic space in certain tiny glands or funnels opening into the two kidneys, according 

 to Sassi. In Placuna these laterally-placed "funnels" appear to be paralleled by a 

 conspicuous fleshy glandular body (accessory excretory organ) on each side, opening 

 respectively into the right and left nephridia. 



Tendency towards further modification in organisation suggested. From the 

 greater development of the left auricle considered in conjunction with the exceptional 

 width of the inter-auricular channel, it seems to me that there exists in Placuna a 

 well-marked tendency towards the eventual modification of the right auricle in such 

 manner that, instead of carrying the blood it receives to the ventricle, it will cease at 

 last to be connected organically therewith and instead will pour its blood stream 

 wholly into the left auricle by way of the further widened inter-auricular channel. 

 This theoretical deduction of the probable results is depicted in fig. 18, which, however, 

 is not an imaginary diagram, but an outline sketch of a similar actual, though 



1 This blood is: probably oxygenated to some extent even though it does not pass through (he 

 gills, as the pallial membranes are so tenuous as to be capable of allowing some exchange of gases 

 between the water without and the blood which (lows within. In this way the wide expanse of 

 free pallium in J'lin-nna may be considered in the nature of an accessory branchial organ. 





