THE NAUTILUS. 21 



part of a filament is the blood space. This blood space may be 

 crossed by an irregular loose network of web-like threads. 

 These probably serve to prevent the membrane from collapsing. 

 In the ventral part of the gill, except at the ventralmost part, 

 the blood spaces of the two lamellae are kept separate as is shown 

 (Figs. 5, 7 and 11). 



At irregular intervals adjacent filaments are joined by inter- 

 filamentary junctions (Figs. 7, 8 and 11). Small ribbon-like 

 bands of fibrous chitin may join several filaments for a short 

 .space. These by holding the filaments together give definite 

 shape to the lamellae which would otherwise be a tangle of 

 tubes. Another type of junction (Fig. 8) is made by the direct 

 fusion of the elements of two adjacent filaments. This is the 

 more common form of junction in the dorsal part of the gill. 

 At the most dorsal part the filaments lose their identity entirely 

 and fuse to form large blood spaces. 



Between the filaments are water spaces which communicate 

 with the mantle chamber on the outside and the cloacal cham- 

 ber on the inside. Water is kept flowing from the mantle cham- 

 iDer to the cloacal chamber and the excurrent siphon by cilia. 

 The outer surface of the filaments is covered with short cilia, the 

 sides have a narrow row of longer ones. 



Circulation of the Blood. The most important function 

 of the gills is the purification of the blood. Blood leaves the 

 ventricle by two main arterial trunks, the one supplying the 

 anterior and the other the posterior part of the body. These 

 vessels end in blood spaces which have no definite wall. The 

 spaces of the greater part of the body pour their blood into the 

 inner lamella of the inner gill (Fig. 5). Passing first ventrally 

 in this lamella, it turns at the bottom of the gill and comes up- 

 ward through the outer lamella. In the dorsal part of this 

 lamella the filaments fuse to form a large sinus which becomes 

 the auricle and empties into the ventricle. The outer gill de- 

 rives its supply of blood from the mantle and such parts of the 

 body as are near by. Blood enters the outer lamella, crosses to 

 the inner lamella and enters the heart by the same channel 

 that carries blood from the inner gill. It should be noted that 

 the brood pouches are admirably located. For they are bathed 



t 



