IO2 



ANIMAL BIOLOGY 



times said to be hatchet-shaped (Fig. 195). Do you see 

 any resemblance ? Does the foot penetrate deep or shal- 

 low into the sand ? (Fig. 190.) Why, 

 or why not ? 



The food tube of the mussel is com- 

 paratively simple. Behind the mouth it 

 enlarges into a swelling called the stom- 

 ach (Fig. 193). The bile ducts of the 

 neighboring liver empty into the stomach. 

 The intestine makes several turns in the 

 substance of the upper part of the foot, 

 and then passing upward, it runs ap- 

 proximately straight to the vent (or anus), 

 which is in the wall of the exhalent 

 siphon. The intestine not only runs 

 through the pericardial cavity (celome) 

 surrounding the heart, but through the 

 ventricle of the heart itself (Fig. 196). 



The kidneys consist of tubes which 

 open into the pericardial chamber above 



FIG. 195. MUSSEL. From 

 below. Level cut across 

 both shells. 



Se, palp; P, foot; O, mouth; 

 G, liver; Gg, Vg, Pg, gan- 

 glia. 



and into the gill chamber below (Neph., 



Fig. 193). The tubes are surrounded by 



numerous blood vessels (Fig. 198) and 



carry off the waste matter from the blood. 

 The nervous system consists of three 



pairs of ganglia and nerves (Fig. 197). 



The ganglia are distinguishable because of 

 ~ their orange color. The pedal 



ganglia on the front of the foot 

 are easily seen also ; the vis- 

 ceral ganglia on the posterior 

 '/* adductor muscle may be seen 

 without removing the mussel 



from the shell (Fig. 193). The reproductive organs 

 open into the rear portion of the gill cavity (Fig. 193). 



The sperms, having been set free in the water, are drawn into 



the ova by the same current that brings the food. The eggs 



\ 



FIG. 196. -- HEART OF 

 MUSSEL, with intestine 

 passing through it. 



