44 



skeleton, and the point does not seem to be easily settled. 

 Eridewood (12) was not able to obtain any conclusive 

 evidence, and Pelseneer (7 ) and Janssens (lOj state that 

 the layer does not exist, while Kellogg (5), Menegaux (17) 

 and Sluiter atHrni that it is present. It is interesting to 

 note that Kellogg (5) states that in Pecten irradians the 

 iutrafilamentar septum is endothelium and not chitin, and 

 in many cases in P. maximus the same appearance is seen. 

 I should not, however, care to aflirm that an endothelial 

 lining exists. 



With regard to the function oi the intratilamentar 

 septum, a very plausible suggestion has been made that 

 Avhen the ascending filaments are not in organic connec- 

 tion, the blood circulating in them must come back again 

 to the gill axis, and the septum keeps the two currents 

 distinct, wdiereas when the filaments are united there is 

 only one current running one way or the other in each 

 filament. 



Ridewood (12) has shown that the septum occurs, 

 however, both in forms with the ascending filaments in 

 organic connection and in those without. 



Further, as will be shown later, the ordinary 

 filaments only communicate with the efferent vessel, and 

 I am inclined to believe, therefore, that what circulation 

 there is in the ordmar}^ filaments is simply a current 

 down the filaments which becomes slower, stops, and 

 then returns by the same channel as observed by Kellogg 

 in Area. It is much more likely, as Drew suggests, that 

 the septum is a brace to keep the filament from swelling 

 laterally owing to the pressure of the blood, and in this 

 way becoming circular and obstructing the floAv of water 

 between the filaments. 



At the iuterlamellar margin of the filaments, and 

 lying between the chitin skeleton and the epithelium, is a 



