HORNELL— ANATOMY OF PLACUNA 71 



In most Lamellibraiichs the anterior extremities of the two ctenidia are separated 

 for some distance by the base of the foot and the adductor portion of the visceral mass. 

 Tn Placuna the palps extend to such a distance ventrally that the anterior apices of 

 the gills are attached well below the foot and on a level with the ventral edge of the 

 visceral mass (fig. 12). Hence the inner margins of the gills meet in the median line 

 immediately ventral to the palps ; save for this inconsiderable divergence the two 

 ctenidia run conjoined throughout their full length. Anteriorly the pallial mesentery 

 is short ; it widens quickly, and for the major portion of its course is from two to three 

 times the depth of the gills. In its posterior third it narrows slightly, while the 

 combined terminal portion of the gills is free from its attachment for a short 

 distance. 



The axis of each ctenidium is hollowed out into a large irregularly tubular cavity, 

 the efferent branchial vessel {Br.eff.), which receives purified blood from the filaments 

 to convey to the trunk leading to the heart. In the floor of this sinus on either side 

 runs a short muscular cylinder or cord, continuous from the anterior to the posterior 

 apex of the gill. These two muscular cords function as branchial retractors, shortening 

 the gills upon contraction and drawing them inwards, an action in which they are 

 assisted by fine muscle fibres which radiate outwards within the branchial mesenteries. 



Unlike the typical arrangement of parallel afferent and efferent vessels within the 

 branchial axis, no second or afferent vessel is to be seen in the gill axis of Placuna. 

 Such vessel is here represented by a well-marked longitudinal blood trunk running in 

 the mesentiery at some considerable distance from its base or attachment (figs. 4, 12 and 

 22, C.br.aff.). Connection between this vessel and the branchial filaments is effected 

 through the intermediary of a network of sinal spaces (fig. 4, Br.ms.pl.) in the tissue 

 of the mesentery ; the arrangement is practically identical with that present in 

 Anomia. 



To understand the significance of this departure from the more normal arrangement 

 as seen in such forms as Margaritiftra and Cardium, it is necessary to recognise the 

 mesentery as morphologically the proximal portion of the primitive ctenidial axis, a 

 portion which has undergone a profound change of form ; the conversion of a simple 

 vascular ridge along the inner side of the axis into a deep sheet of tissue has induced 

 a change from a system where extremely short channels connect a proximate afferent 

 trunk with the individual filaments, to one where a complex and extensive network 

 of ill-defined sinal spaces is intercalated between the filaments and a far removed 

 afferent trunk. It would seem that the short afferent branches of the one have in part 

 been modified into this peculiar sinal plexus. 



The effect of this mesenteric development is to add materially to the blood- 

 purifying area ; it has a distinct accessory branchial function, and must assure a partial 

 oxygenation of the blood stream prior to it passing into the gill filaments, thereby 

 facilitating their branchial function. The very extensive development of the 



