106 



MU.SSEL.S, CLAxMS, OYSTERS, SNAILS, SQUIDS 



Inhalent 

 siphon 



edges of the valves, l)ut 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 siphons, which pro- 

 ject between the edges of the valves when the mussel rests 

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

 is always passing in through the lower, or inhalent siphon, 



and out through the 



Exhalent siphon Inhalent i 7 j • 7 



^^ ■ ■ upper, or exhalent siphon 

 (Fig. .5(1). 



The gills and respira- 

 tion. — On either side of 

 the posterior i)art of the 

 body, inside the mantle, 

 is a pair of gills. They 

 hang susix'nded with the 

 lower edges free and pre- 

 sent a ribbed, or striated 

 appearance. Each gill ha,s 

 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, 

 ciliated 0]ienings through 

 which the water flows. 

 Moreover, canals run up 

 and down inside of the 

 thin walls of the gills for conveying the blood (Fig. 5G, B). 

 The outer walls of the outer gills are attached to the 



Mbiith. 



^■- ^^'~-'~"-J' ^ O^^J~--j 4- J~''--''^~^ - 



Blood 



Wati^r 



Fig. 56. — Diagram ai a clam and 2:iortion 

 of a gill. 



