102 



AXIMAL 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 (celonie) 

 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 

 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 ga?ig/ia 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. 195.— Missel. From 

 below. Level cut across 

 both shells. 



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

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

 glia. 



Fig. 196. — Heart of 

 Mi SSEL, with intestine 

 passing through it. 



Fig. 197. 



