THE LAMELLIBRANCHIA 231 



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 



Transverse section of Cuspidaria, taken through 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 



