IntrodMction to Animal Morphology. 263 



the auricle. In the organ of Bojanus the blood 

 becomes mixed with water, and thence passes by 

 channels (branchial arteries) to the gills, and is re- 

 turned by sacculated bronchial veins into the auricle. 

 The heart beats 5-10 times (Unio), or 20-30 times 

 in a minute, and continues to act after the animal is 

 taken out of the water. The sinus venosus and 

 bloodvessels have no propelling power. 



The gills are lamellar, placed longitudinally on 

 each side between the mantle and the foot, having the 

 free borders directed ventrally. They are usually 

 symmetrical (except in inequivalve genera, particu- 

 larly in Anomia), and in two pairs on each side 

 (Corbis, Lucina, and many other Lucinidse, have but 

 one pair) ; of these, the external are the smaller and 

 the later in development. Each gill lamella consists 

 of two layers of comb-like* filaments, which are free 

 (Arcidae, Pecten, Trigonia, Spondylus), or united by 

 horizontal bands into a continuous multifenestrated 

 plate, supported on a framework of chitin rods, ar- 

 ranged in rows behind each other, and richly ciliated 

 on the surface. The layers are more or less united, 

 directly or by a connecting membrane. The inner 

 gill lamella may adhere to the foot, leaving an inter- 

 nal branchial channel between them ; the outer la- 

 mella may be unilaminar; the space between the gill- 

 lamellae is the interbranchial chamber. The ciliary 



two from the viscera, and one from the mantle. Its blood is poured into 

 the organ of Bojanus. 



* A lamellibranch is aptly compared by Keferstein to a book whose 

 back is represented by the hinge hne, its cover by the shell valves, its fly 

 leaves by the mantle lobes, its second and third pages at each side by the 

 gill lamellae, and its interior by the \'isceral sac and foot. In Anomia the 

 sickle-lilce gills are removed from the visceral cavity, and do not extend 

 as far as usual. 



