HORNELL ANATOMY OF PLACUNA 73 



it close to the parieto-splanchnic ganglion. The vessel first passes outwards for 

 about one-fourth the depth of the mesentery and then divides into a short anterior 

 branch proceeding towards the anterior apex of the ctenidium and a long posterior 

 branch which passes to the posterior extremity. Both branches keep well to the 

 inner side of a line that would divide the mesentery into a proximal and a 

 distal half. 



From each of these vessels a large number of branches are given off which 

 divide and ramify throughout the distal section of the mesentery to form an 

 anastomosing network of sinal spaces (Sr.ms.pl.). Along the outer margin of 

 this plexus and fed by it, a series of regularly disposed and slightly swollen 

 " capillaries " arise, which pass blood into each branchial filament. This current 

 of blood flows outwards within the smaller or afferent of the two parallel cavities 

 within the filament, returning by the larger or outer tube to empty into the wide 

 efferent vessel running along the ctenidial axis (figs. 4, 12, and 24, Br.eff.}. This 

 vessel carries the blood forwards and upwards and is directly continuous with 

 a short well-marked wide vascular trunk the common efferent or branchio-cardiac 

 vessel (fig. 24, C.br.eff.), running at right angles to the anterior portion of the 

 branchial axis and connecting the anterior extremity of the ctenidium of its 

 respective side with the auricle of the same side, appearing indeed as an anterior 

 prolongation of the latter. 



Oxygenation takes place partly during the flow of the blood through the 

 mesenteric sinal plexus, partly during the flow through the branchial tubules. 



Each ctenidium derives its nerve supply from a trunk which arises from the 

 parieto-splanchnic ganglionic mass, and which runs within the mesentery some 

 distance from and parallel with the ctenidial axis (figs. 10 and 11, N.br.). 



FUNCTIONS OF THE BRANCHIAE. Apart from the oxygenating function which 

 in Placuna as in Anomia is shared by the branchial mesenteries, the gills of 

 Placuna serve two well-marked mechanical functions, (a) the capture of food particles, 

 and (b) their conveyance to the mouth. The rhythmic lashing of the cilia clothing 

 the outer faces of the filaments causes an indraft of water into the shell which not 

 only serves to oxygenate the blood passing through the branchial tubules and 

 mesenteries but ensures a continual stream of food particles being brought in. 

 The close approximation of individual filaments and the fringes of cilia along their 

 sides then come into play to intercept and strain out these particles, which are 

 then caught up by certain cilia and propelled first to the crest (ventral edge) 

 of the branchiae and thence along this crest direct to the labial palps and 

 mouth. 



To properly perform these varied functions the cilia of the filaments have under- 

 gone specialisation and peculiar localisation. We have noted in each filament lour 

 distinct ciliated tracts marked off by the length and disposition of the cilia, (a) a broad 



