160 CONVERGENCE IN MINUTE STRUCTURE 



in this species. Similar relations have been 

 described by Dakin 1 in another species of scallop, 

 Pectcn maximus, and the unpaired condition of 

 the organ is obviously a character of the genus 

 Pec ten. 



In the Oriental disc-shaped bivalve, P^tna 

 placenta, belonging to the family Anomiidae, 

 only one sensory abdominal ridge occurs. It is 

 defined by the presence of brown pigment, and 

 is placed upon the edge of an adrectal ligament 

 or cutaneous fold which unites the rectal complex 

 with the right mantle lobe at about the level of 

 the posterior insertion of the suspensory ligament 

 of the right ctenidium. The pallial circulation 

 of Placuna is characterised by the presence of 

 a pair of ascending pallial vessels, which arise 

 from the circumpallial arteries at the postero- 

 ventral border of each mantle lobe and pass 

 obliquely upwards towards the posterior end of 

 the suprabranchial region. On approaching the 

 latter their walls are usually inflated, after the 

 valves have been separated, to form an elongated 

 oblong or elliptical body which is non-contractile. 

 I do not know the precise significance of this 

 congestion of the walls of the ascending pallial 

 vessels of Placuna, but it concerns the vascular 

 system and has nothing to do with the adrectal 

 sensory organ. 



The abdominal sense-organ of Placuna con- 



1 Memoir on Pecten. Liverpool, 1909. 



