SQUID. 107 



of this will be found a large centre, the cerebral ganglion, 

 while on the ventral side two somewhat smaller ganglia 

 occur. The anterior of these is the pedal ganglion, and 

 from it nerves can be traced running into the arms. 

 The posterior is the visceral ganglion. The oesophagus 

 passes between the cerebral on the one hand and the pedal 

 and visceral ganglia on the other. In one half of the head 

 demonstrate by dissection that these ganglia are con- 

 nected. Except that the ganglia are much closer together 

 and the connections correspondingly 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. Draw a diagrammatic section of the eye. 



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. 



