ANATOMY OF A FRESH-WATER MUSSEL. 49 



one valve of the shell to the other. In Anodonta they are equal 

 in size (Isomya); in Dreissensia polymorpha the anterior is smaller 

 than the posterior adductor muscle (Heteromya). Some Lamelli- 

 branchs have only one adductor muscle, and are then called 

 " monomyaries ; " those with two adductor muscles are termed 

 " dimy aries." 



6. The anterior and posterior retractor pedis muscles, near the 

 anterior and posterior adductors. 



7. The protractor pedis muscle, behind the anterior adductor. 



8. The pallial muscle, attaching the mantle to the pallial line. 



9. The inhalent aperture, bounded with tentacular fringes at 

 the hinder end of the body. 



10. The exhalent or cloacal aperture, immediately dorsal to the 

 inhalent aperture. 



Thus Keferstein's analogy of the general structure of a Lamelli- 

 branch to a book is appropriate. The cover of the book is the 

 valves, the back is the hinge-line, the fly-leaves are the mantle- 

 lobes, the second and third pages on each' side are the gills, and 

 the interior of the book is represented by the visceral mass and 

 foot. 



THE MANTLE, MANTLE-CAVITY, AND GILLS. The mantle- 

 lobes are symmetrical. They are continuous with the body-wall 

 above, below they are free. The epithelium on their internal 

 surfaces is ciliated. Between them lies the mantle or pallial 

 cavity, divided by the base of the gills into an upper or supra- 

 branchial chamber, and a lower or infra-branchial chamber. In 

 this pallial cavity lie the gills, foot, labial palps, and the greater 

 part of the visceral mass. The gills or ctenidia consist of a pair 

 of plates lying on each side of the foot. Each plate is made up 

 of a descending and an ascending lamella, the descent and ascent 

 being from a ridge on the body-wall termed the gill-axis. Each 

 lamella is composed of a number of gill-filaments which have 

 become fused together so as to form a trellis-work-like structure. 

 It will be best to trace these gill lamellae from the gill-axis. From 

 the gill- axis two descending lamellae proceed, one to form the 

 inner lamella of the outer gill, the other to form the outer lamella 

 of the inner gill. These, descending for some distance, at last 

 turn sharply upon themselves, and the inner lamella of the outer 



4 



