106 



MUSSKLS, CLAMS, OYSTERS, SNAILS, SQUIDS 



Exhalent siphon 



edges of the valves, but the mantle is fastened, a little way 

 from the edge, to each half of the shell along the pallial 

 line, already noted. At the posterior end of the shell 

 the mantle forms two short tubes, the siplimis, which pro- 

 ject between the edges of the valves when the mussel rests 

 undisturbed (Fig. 5(1). In this position a current of water 

 is always passing in through the lowei', or inhalent siphon, 



and out through the 

 :p/m:i uppcr. Or exIicUent siplwn 



(Fig. 56). 



The gills and respira- 

 tion. — On either side of 

 the posterior part of the 

 body, inside the mantle, 

 is a pair of gills. They 

 hang suspended with the 

 lower edges free and pre- 

 sent a ribljed, or striated 

 app(>arance. Each gill has 

 the form of a long narrow 

 bag or trough open above 

 and divided into compart- 

 ments by cross-partitions. 

 In addition, the sides of 

 the bag, or gill, are per- 

 forated with many small, 

 ciliateil openings through 

 which the water flows. 

 Moreover, canals run up 

 and down inside of the 

 Ihiii wnli.s of the gills for con\'c\ing the blood (Fig. .56, B). 

 Till' iiuter walls of (hi' outer gills are attached to the 



tloiilh 



Blood 



Water 



l^'io. .50. 



- Diagram of a. (-Iain and portion 

 of a gill. 



