. Beginners' Zoology . Are they nearerthe ventral or the dorsal surface?The points of attachment travelleddownward and farther apart as theanimal grew (see Fig. 191). Higher than the larger scars are small scars, or impressions, wherethe protractor and retractor muscles that extend and drawin the foot were attached. The muscular/<?^/ extends downward in the middle, half-way between the shells (Fig. 193). On each side of thefoot and behindit hang downthe two pairs ofgills, the outerpair and the in-ner pair (). They maybe compared tofour V-shapedtroughs withtheir sides full of holes. T
. Beginners' Zoology . Are they nearerthe ventral or the dorsal surface?The points of attachment travelleddownward and farther apart as theanimal grew (see Fig. 191). Higher than the larger scars are small scars, or impressions, wherethe protractor and retractor muscles that extend and drawin the foot were attached. The muscular/<?^/ extends downward in the middle, half-way between the shells (Fig. 193). On each side of thefoot and behindit hang downthe two pairs ofgills, the outerpair and the in-ner pair (). They maybe compared tofour V-shapedtroughs withtheir sides full of holes. The water enters the troughsthrough the holes and overflows above. Is there a markeddifference in the size of the two pairs of gills ? A ki:id of. POSTS ADP«MUS., Fig. 193.— Anatomy of Mussel. (Beddard.) MOLLUSCS lOI chamber for the gills is made by the joining of the mantleflaps below, along the ventral line. The mantle edges areseparated at two places, leaving openings called exJialetitand inlialent siphons. Fresh water with its oxygen, propelled by cilia at theopening and on the gills, enters through the lower orinhalent siphon, passes between the gills, and goes to anupper passage, leaving the gill chamber by a slit whichseparates the gills from the this passage, see arrow(Fig. 194). The movement ofthe water is opposite to the waythe arrow points. After goingupward and backward, the wateremerges by the exhalent gills originally consisted ofa great number of are now united,.but notcompletely so, and the gills stillhave a perforated or latticestructure. Thus they present alarge surface for absorbing oxy-gen from the water. The mouth is in front of the foot, between it and theanterior addu
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