SENSE ORGANS— ALIMENTARY SYSTEM. 



487 



ventral (motor) roots. The first sixteen or eighteen nerves 

 form the brachial plexus, converging and uniting in a trunk 

 which supplies the pectoral fin. 



The sympathetic system consists of a longitudinal ganglion- 

 ated cord along each side of the vertebral column. The 

 ganglia of these cords are connected with the spinal nerves. 



Se?ise Organs. 



(a) The Eyes [see p. 445). The iris has a beautifully fringed upper 



margin. 



(b) The Ears (see p. 444). The vestibule is connected with the sur- 



face by a delicate canal — the aqueductus vestibiili — a remnant 

 of the original invagination. A small part of the wall of the 

 auditory capsule is covered only by the skin forming a kind of 

 tympanum. Within the vestibule are calcareous otolithic par- 

 ticles surrounded by a jelly. 



(i") The Nasal sacs are cup-like cavities with plaited walls. 



(d) The Sensory tubes are best seen on the ventral surface, where they 

 lie just under the skin. They end in ampuUce, containing 

 sensory cells. 



Alimentary System. 



The mouth is a transverse aperture ; the teeth borne by 

 the jaws are numerous, and those worn away in front are 

 replaced by fresh teeth from be- 

 hind ; naso-buccal grooves connect 

 the nostrils with the corners of the 

 mouth ; the spiracles, which open 

 dorsally behind the eyes, communi- 

 cate with the buccal cavity; from the 

 gullet five gill clefts open ventrally 

 on each side ; the stomach lying 

 rather to the left is bent upon itself; 

 the large brownish liver is trilobed, 

 and has an associated gall bladder, 

 from which the bile duct extends 

 to the duodenum — the part of 

 the gut immediately succeeding the 

 stomach ; the whitish pancreas lies 

 in the duodenal loop between 

 stomach and intestine, and its duct 

 opens opposite the bile duct ; the intestine contains an in- 

 ternal spiral fold — a membrane which increases the absorp- 



FlG. 159. — .Spiral valve 

 of Skate. (After T. J. 

 Parker.) 



