Page:First course in biology (IA firstcourseinbio00bailrich).pdf/338

times said to be hatchet-shaped (Fig. 195). Do you see any resemblance? Does the foot penetrate deep or shallow into the sand? (Fig. 190.) Why, or why not?

Fig. 195.—Mussel. From below. Level cut across both shells.

Se, palp; P, foot; O, mouth; G, liver; Gg, Vg, Pg, ganglia.

Fig. 196.—Heart of Mussel, with intestine passing through it.

Fig. 197.

The food tube of the mussel is comparatively simple. Behind the mouth it enlarges into a swelling called the stomach (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 approximately 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 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 visceral 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