196 THE BIOLOGY OF THE AOTMAL 



Turn back one of the lobes of the mantle and 

 note 

 g. The pallial cavity — How is it formed ? "What does 

 it contain ? Note its division into a large ventral 

 cavity, the TbrancMal cavity, and a much small- 

 er dorsal cavity, the hinder portion forming the 

 cloacal cavity. 



h. The abdomen. — What is its relation to the muscles? 

 What is its shape ? Color ? Is it bilaterally sym- 

 metrical ? Is there a head ? Are there any bones 

 in the body ? How is it held in the sheU ? 



Projecting from the lower margin of the ab- 

 domen is 



i. The foot. — What is its position with regard to the 

 abdomen? To the paUial lobes? To the gills? 

 What is its shape? Size as compared with the 

 abdomen ? Color ? Structure ? To what is it 

 attached ? In what direction does it project ? 



_;'. The gills or gill-plates. — What is their position 

 with regard to abdomen, foot, siphons, and man- 

 tle ? Number ? Shape ? Size as compared with 

 the mantle folds ? Color ? How attached to the 

 body proper and the other parts ? Cut along the 

 line of attachment of the outer gill to the mantle. 

 Notice that a canal, the supra - branchial or 

 cloacal chamber, is thus laid open. Notice also 

 running down into the gill between its two faces 

 a number of fine tubes, the water-tubes, sep- 

 arated from one another by partitions. These 

 tubes are closed at the lower margin of the gill, 

 as may be proved by passing a fine bristle into 

 a tube. Carefully split open a giU, and examine 

 both surfaces with a hand-lens. Note that each 



