GENERAL AXATOMY OF THE SQUID. 363 



2. On the dorsal median line, the upper end of the 

 pen (n) in its sac. 



3. Opposite this, on the dorsal surface of the head, the 

 neck cartilage {!). 



4. Ventral to this the retractor muscle of the head (a). 

 On the middle line, near the dorsal surface of the head, 

 the oesophagus (c), with a salivary duct on each side of it. 



5. Ventral to the oesophagus the cross section of the vis- 

 ceral ganglion (d), which consists of a central axis of white 

 matter, and a thick peripheral layer of gray matter. 



6. Around the oesophagus and visceral ganglion the ce- 

 phalic cartilage {g, g), which differs in form in different 

 sections. It forms a concave tray or l)ox, which supports 

 the brain, and folds up around it at its edges. 



7. Ventrally to the visceral ganglion tlie ear-capsule (e), 

 — a large hollow with vertical ridges of cartilage project- 

 ing into it from the sides. In a favorable section the ear- 

 capsule will 1)6 seen to contain a second capsule of lai^e 

 ciliated cells, outside which are scattered ganglion cells. 

 In a section through the anterior edge of the ear-capsule 

 the auditory ner\'e may be seen to pass from the visceral 

 ganglion into the ridge on the dorsal surface of the cap- 

 sule. 



Other sections may show the nerves which pass from 

 the visceral ganglion to the siphon, and to the ganglia 

 stellata. 



8. On the ventral median line between the ears and the 

 siphon, notice the single anterior vena cava, which has 

 been formed by the union of the two which are found in 

 more anterior sections. 



9. Notice the cross section of the siphon, ^vith its two 

 cartilages (//) fitting around the cartilaginous ridges (i) on 

 the inside of the mantle. 



