THE LAMELLIBRANCHIA 231 
op ee a 
connective tissue. This thickening is specially well developed on 
the internal sides of the lamellae in the Anomiidae, Arcidae, and 
Trigoniidae (Filibranchia, in which the filaments are very loosely 
held together), while in the rest of the Lamellibranchia the thicken- 
ing is most developed on the external or ventral side. 
The cavity of the branchial filaments is divided lengthwise by a 
connective tissue septum in the Anomiidae, Arcidae, and in Pecten. 
The afferent branchial vessel runs in the dorsal half of the axis in 
the Protobranchia, and consequently the blood enters each filament 
on the dorsal side and passes to the ventral side to enter the efferent 
branchial vessel leading to the auricle. Thus in each filament 
there are two currents running in opposite directions, but continuous 
Fiq@. 211. 
Transverse section of Cuspidaria, taken throngh the heart. I, branchial septum ; II, 
branchial nerve ; III, sphincter of branchial orifice ; IV, mantle ; V, septal orifice ; VI, posterior 
retractor of the foot; VII, testis; VIII, intestine; IX, accessory genital gland; X, visceral 
commissure ; XI, ventricle; XII, pericardium ; XIII, auricle; XIV, kidney. 
with one another. The same thing may be seen in the narrow and 
still independent filaments of the Anomiidae and Arcidae, but when 
the cavities of successive filaments are put into communication with 
one another along the free borders of the reflected lamellae, there is 
but one current in each filament running from the afferent vessel 
(whose position is variable) to the efferent vessel common to the 
two gill-plates. 
In addition to their respiratory function, the gills are accessory 
to the function of alimentation. The action of their cilia produces 
a constant current from behind forwards, which carries particles 
suspended in the water towards the labial palps, and consequently 
towards the mouth. In some rare cases—for example, in Mytilus— 
an accessory respiratory apparatus is developed in addition to and 
