150 MOLLUSCA. 



where the body joins the mantle. They send nerves to the man- 

 tle and are joined to ganglia in the head (the infra-esophageal) 

 by connectives. Why does the mantle need such large, special 

 ganglia? Other small ganglia are situated in the body, but the 

 large and important ones are grouped in the head, where they are 

 supported and protected by cartilages. 



With a razor make a median sagittal section of the head of 

 a squid and notice: 



1. Dorsal to the esophagus a rounded mass, the supra-eso- 

 phageal ganglion, which is supposed to represent the fused cere- 

 bral ganglia. 



2. Ventral to the esophagus the elongated infra-esophageal 

 ganglion, which is supposed to represent the fused pedal and 

 visceral ganglia and (together with the masses that connect the 

 supra- and infra-esophageal ganglia around the esophagus) the 

 pleural ganglia. 



3. The anterior prolongation of the infra-esophageal gan- 

 glion to form the pro-pedal portion, which supplies nerves to the 

 arms. 



4. The small supra-huccal ganglia, lying dorsal to the esoph- 

 agus, and a little further anterior than the ends of the pro-pedal 

 portion. These are joined by connectives with the supra- 

 esophageal ganglia. 



5. The infra-huccal ganglia, about the same size as, and lying 

 ventral to, the supra-buccal ganglia, and joined with them by 

 connectives that run around the esophagus. 



Draw a figure of a sagittal section of the head. 



Two large ganglia, the optic ganglia, lie against the eyes 

 and will be seen in cross-sections of the head that will be studied 

 later. A dissection of one side of the head will show one. 



Open the animal along the mid-dorsal line and find the pen 

 which is embedded in the mantle. After exposing it for its 

 full length, turn the flaps aside and see that it lies in a pocket. 

 It probably represents a modified shell that has become entirely 

 inclosed by the mantle. What is its function? 



Pull the pen out of the mantle and draw it. 



