SENSES OF CEPHALOPODS. 25 



chial cephalopods. In these mollusks there are two very large 

 eyes of spheroidal form, lodged in the lateral parts of the head. 

 These organs are composed of a transparent cornea, let into the 

 skin, and sometimes protected by a fold, like an eyelid, a crystal- 

 line lens, a vitreous body, a retina, a choroid coat, a sclerotica, 

 &c., nearly the same as in vertebrate animals. In the tetrabran- 

 chial cephalopods the eyes are borne on projecting peduncles, and 

 consist of a cavity only, the black interior of which contains a 

 retina, and receives the luminous rays through a circular open- 

 ing. No auditory apparatus has yet been discovered in the first 

 of these two families. We find in the cephalic cartilage of the 

 latter, two small cavities, closed on all sides, except at the point 

 through which the nerve penetrates ; they lodge a membranous 

 vesicle, and are filled with a peculiar fluid, containing a small 

 stone. This, as we see, is a still more simple arrangement than 

 that of fishes ; for the whole apparatus is reduced to a vestibule 

 and a nerve only. 



12. The nervous system of the cephalopods is more compli- 

 cated than that of the other mollusks, and the different ganglia, 

 grouped around the oesophagus, have a greater tendency to be- 

 come confounded in a single mass. The medullary collar, thus 

 formed, is composed of three pairs of ganglia, namely : the 

 cephalic ganglia (fig. 11, c), the tentacular ganglia (), and the 

 thoracic ganglia (g) ; the two first pairs are placed above the 

 oesophagus, and, by joining the first, form a double collar around 

 this tube; sometimes they are very distinct, but at others, they 

 are almost entirely confounded. The cephalic ganglia give rise 

 laterally to two large nervous cords, which, from their origin, 

 enlarge into ganglia, and then furnish the optic nerves ; from 

 this species of brain, nerves also arise, which go to the mouth, 

 and sometimes form around this opening a new collar, fur- 

 nished with two pairs of small ganglia (fig- 5, page 13). The 

 tentacular ganglia afford origin to the nerves of the feet or ten- 

 tacula, which, before ramifying on these organs, also present 

 ganglionic swellings. Finally, the thoracic ganglia, sometimes 

 very distinct from the preceding, and at others confounded with 

 them (see fig. 6, page 17), furnish many nerves, the most import- 

 ant of which are, 1st, The two nerves of the viscera (fig. 11, fl), 

 which in their course present a pair of ganglia and distribute 

 their branches to the branchiae, the heart, stomach, &c. ; 2d, The 

 nerves of the mantle, which, in the dibranchial cephalopods, 



11. Where are the eyes situated in cephalopods? Are cephalopods ca- 

 pable of hearing ? 



12. What is the general character of the nervous system of cephalopoda ' 



3 



