PISCES. 419 



absent in Dipnoi. The facial and auditory have a common root, as in 

 many Amphibia and Reptilia. The glossopharyngeus is a separate nerve, 

 with a separate foramen in the skull. The vagus has several roots (five at 

 least) in Elasmobranchii and Dipnoi ; in the latter and some of the former 

 independent ventral roots as well. There is no spinal accessory, and the 

 hypoglossus is represented by spinal nerves. The fifth, the facial in Teleostei, 

 the glossopharyngeus and vagus have intracranial dorsal branches ; that of 

 the fifth in Teleostei extends down the body near the median dorsal line ; 

 it is connected both to the spinal nerves and the lateral line branch of the 

 vagus, and supplies the muscles and integument of the dorsal fins. The 

 lateral line branch of the vagus lies beneath the integument, except in 

 Elasmobranchii, where it is deeply placed in the septum, between the 

 dorso- and ventro-lateral muscles. The anterior roots of each spinal nerve 

 pass in Elasmobranchii through or behind the neural arches, the posterior 

 roots through or behind the intercalary pieces ; they therefore alternate in 

 position. In Acipenser the roots of the spinal nerves of one side of the 

 body alternate in position with the roots of the other side. The spinal 

 ganglia in Teleostei sometimes lie within the spinal canal, a most unusual 

 position. The anterior end of the sympathetic system is connected to the 

 vagus in Elasmobranchii and chondrostean Ganoidei, but in Teleostei to the 

 third nerve, as in higher Vertebrata. 



Tactile barbules are found on the head of the Sturgeon, the Cods, 

 many Siluroidei. The nasal sacs are placed ventrally in front of or near 

 the angles of the mouth in Elasmobranchii and Holocephali, on the dorsal 

 aspect of the face in front of the eyes in Ganoidei and Teleostei. They 

 have usually an anterior and posterior opening, the former being occasion- 

 ally, as in Polypterus and some Teleostei, prolonged into a tube. A channel 

 runs down to the angle of the mouth from the nasal chamber in Rays and 

 some Sharks. In Dipnoi the anterior aperture lies just in front of the 

 mouth and outside it, the posterior within it externally to the vomerine 

 teeth. The mucous membrane is disposed in folds. Certain Plectognathi 

 ( Tetrodoit] are said to have solid olfactory tentacles. Protopterus alone has 

 an incomplete fenestrated nasal capsule. The eyes have no glands, and 

 lids are represented by slight folds, most marked in Elasmobranchii, some 

 of which, e. g. Mtistefas, Carcharias, have a third eyelid or nictitating 

 membrane. The bulb is imbedded in a fatty gelatinous tissue traversed by 

 connective tissue fibres. It is small in Dipnoi, rudimentary in some deep- 

 sea, cave- or mud-inhabiting Teleostei, but is usually large. In Pleuro- 

 nectidae (Soles) both eyes come to be placed on the same side of the head. 

 The cornea is flat, the sclerotic generally strengthened by calcifications. 

 An argentea a silvery or golden cellular layer, the colour due to minute 

 crystals lies externally to the choroid in most Teleostei, and extends 

 into the iris. The latter is absent in Dipnoi; its musculature is feebly 



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