Introduction to A n imal 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 Lucinida?, 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 maybe unilaminar; the space between the gill- 

 lamellae is the interbranchial chamber. The ciliary 



two from tli- -id one from the mantle. It^ Mood is poured into 



the or 



aptly comparn 1 l>y / 



back i . ilu- shell \.ilves, its lly 



ml and tliiid | : ^de l>y the 



gill lamcll.-r, and l.y the visrer.il sar and foot. In Anomia the 



ickle-likc {,nlls are removed from the visceral c.i\ity, and do nd extend 

 as far as u^ual. 



