256 ELEMENTS OF COMPARATIVE ZOOLOGY. 



section that these ganglia are connected. Except that the 

 ganglia are much closer together and the connections cor- 

 respondingly shortened, are the relations the same as in 

 the clam ? 



Just ventral to the visceral ganglion is an enlargement 

 of the cerebral capsule; this is the ear. Cut into this and 

 notice that it has an irregular cavity. Is there a similar 

 structure on the other side of the head? Sketch the 

 section of the head, showing the ganglia, jaws, lingual 

 ribbon, oesophagus, and ear, in the drawing already made 

 of the alimentary tract. 



Split one half of the head in a horizontal plane, having 

 the section pass through the middle of the eye. In the 

 section thus made study first the eye itself. This is covered 

 externally with a transparent cornea, and inside contains 

 two chambers, separated from each other by the solid lens. 

 The outer chamber in turn is partially divided by a circular 

 fold, the iris. The inner chamber is bounded internally 

 by the retina, the outer surface of which is marked by a 

 thin layer of black pigment. Behind and dorsal to the eye 

 is the optic ganglion, bounded posteriorly by a cartilage wall. 

 Trace the connections of the optic and cerebral ganglia. 



Cut into the dorsal region of the mantle from the outside 

 and find the horny pen. Continue the cutting so that it 

 may be taken out. Sketch it. 



COMPAKISONS. 



With two columns, one for oyster and clam and one for 

 squid, answer the following questions : 



(1) Is there a distinct head ? 



(2) Are there cephalic tentacles ? 



(3) Is there a bivalve shell ? 



