Notice that the teeth on the two valves interlock. What is their 

 function ? 



Draw a 1'ali'c as seen from the inside. 



5. By examining a shell of Mytilus, or Unio, near its margin, 

 the typical three layers of which it is composed can be seen. How 

 is it possible for all three layers to be secreted by the mantle, which 

 lines the inside of the shell ? Can you rind any reason for more 

 than one layer? 



IV. MANTLE. 



This consists of two lobes (one of which is normally applied 

 to the inner surface of each valve of the shell ) , that are united 

 do rally. 



1. The free border of each lobe is thickened and contains muscles 

 which are attached along the palial line. What function do these 

 muscles perform? 



2. The posterior portions of the lobes of the mantle are thickened 

 and united to each other so as to form two tubes (in Unio the 

 ventral tube is formed by contact only), the siphons, through which 

 water passes into and out of the shell. 



3. See how the muscles of the siphons are arranged and attached. 

 Does the attachment bear any relation to the palial sinus? 



V. VISCERAL MASS AND FOOT. 



These portions form the large median mass. The viscera are 

 contained in the dorsal portion. The ventral portion is hard and 

 muscular and forms the foot. 



VI. GILLS. 



These consist of two thin, striated lamellae (gills) on each side 

 of the foot. The inner and outer gills of each side are attached 

 to each other along their dorsal borders, and the outer gill is also 

 attached to the mantle ; the inner one is attached to the visceral 

 mass anteriorly, while posteriorly the inner gills of the two sides are 

 united, thus partially separating an upper cavity, the cloacal cham- 

 ber, from a lower one, the branchial chamber. These attachments 

 occur only at the dorsal borders of the gills, which otherwise hang 

 free in the branchial chamber. The ventral siphon opens into the 

 branchial chamber, while the dorsal siphon opens out from the 

 cloacal chamber. Labial Palps : A pair of small triangular flaps 

 on each side of the visceral mass. The outer palps are united 

 above the mouth, which is situated just posterior to the ventral 



