80 A FRESH-WATER MUSSEL. 



The first and last layers mentioned are secreted by the thickened 

 edge of the mantle, the nacre by the whole outer surface of that 

 organ. 



Exercise 2. Draw the inner surface of the left valve, natural size. 



THE BODY PROPER. You will now observe that the mantle which 

 lines the shell is attached to the dorsal wall of the body and completely 

 envelops it, hanging down on either side like a cloak. 



a) When the edges of the two lobes are brought together a large 



chamber, the branchial cavity, is enclosed. Into this the 

 inhalant siphon opens. 



b) Turn back the mantle lobe, without tearing it loose at any point, 



and note, within the chamber, the wrinkled hatchet-shaped 

 foot projecting from the larger and thicker visceral mass. 



c) On either side of the visceral mass are two thin plate-like organs 



with parallel vertical striations. These are the gills, an outer 

 pair and an inner pair. It will be noted that they extend from a 

 point somewhat anterior to the middle of the visceral mass to the 

 siphonal openings, where the united dorsal edges of all four 

 constitute a horizontal partition between the branchial cavity 

 and a much smaller chamber, the suprabranchial cavity, from 

 which the exhalant siphon opens. 



d) In front of the gills on either side are two triangular flaps, the 



labial palps. 



e) Just back of the anterior adductor muscle is the wide mouth, 



bordered in front by a sort of lip formed of united folds projecting 

 from the outer pair of palps. Similar folds from the inner pair 

 of palps unite back of the mouth. The palps thus resemble, as it 

 were, a pair of military mustaches attached to each lip. They are 

 provided with cilia which aid in sweeping food particles to the 

 mouth. 



f) In front of the posterior adductor and above the attachment of the 



mantle lobe to the body is a triangular chamber with very thin 

 walls. This is the pericardium. 



