376 TRANSACTIONS LIVERVOOL BIOLOGICAL SOCIETY. 
skeleton, and the point does not seem to be easily settled. 
Ridewood (12) was not able to obtain any conclusive 
evidence, and Pelseneer (7) and Janssens (10) state that 
the layer does not exist, while Kellogg (5), Menegaux (17) 
and Sluiter affirm that it is present. It is interesting to 
note that Kellogg (5) states that in Pecten irradians the 
intrafilamentar septum is endothelium and not chitin, and 
in many cases in P?, maaimus the same appearance 1s seen. 
I should not, however, care to affirm that an endothelial 
lining exists. 
With regard to the function of the intrafilamentar 
septum, a very plausible suggestion has been made that 
when 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, whereas 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 ordinary 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 flow of water 
between the filaments. 
At the interlamellar margin of the filaments, and 
lying between the chitin skeleton and the epithelium, is a 
