occupied by perilymphatic or endolymphatic tubes. Cer- 

 tainly the canals to the dorsal field suggest the endolym- 

 phatic ducts of gnathostomes; however, what appear to be 

 external openings of endolymphatic ducts are also present 

 in some of these fossil agnaths. The presence of canalicular 

 extensions of the inner ear would agree with the theory that 

 the ear is a derivative of an accustico-lateralis system. The 

 fields and canals of the fossil cephalaspids represent a spe- 

 cialization characteristic of that group. 



In sharks the anterior vertical canal joins with the hori- 

 zontal canal to form a crus, thus differing from other fishes 

 including Hydrolagus. In the latter the vertical canals form a 

 crus into the base of which the horizontal canal enters. The 

 endolymphatic duct of the shark exits from the capsule 

 directly into a fossa below the skin. There is an endolym- 

 phatic sac opening to the outside at a pore. There are 

 granules associated with the maculae in this group. Hydrolagus 

 has external pores for the endolymphatic ducts. 



In actinopterygians the inner ear has distinct utricular, 

 saccular, and lagenar sacs and usually each has a stony 

 otolith. There is a macula neglecta on the outer wall of the 

 constriction between utriculus and sacculus. In detail of 

 parts in this group, there is much variation. In some, the 

 sacculi are joined across the midline and are associated with 

 a perilymphatic sac which extends out on either side of the 

 anterior vertebrae. This perilymphatic sac lies in contact 

 with a series of four Weberian ossicles, detached and modi- 

 fied processes of the vertebrae — Figure 13-24, which are 

 articulated as a conducting chain. The most posterior of 

 these lies in contact with the air bladder. Vibration of the 

 bladder or, more probably, pressure changes (expansion 



utriculus. 



optic lobes 



-cerebellum 



endolymphatic connective 



iogena 



scaphium 



intercclorium 

 ligament- 



tripus 



-\- — perilymphatic sac 



,^^nw^ 



anterior chamber of 

 swim bladder 



Figure 13-24. Webberian ossicles of Cofostomus commersoni as seen 

 in dorsal view, semidiagrommatic. (After Martin, 1961) 



and contraction) are thus transmitted through these bones 

 and the perilymphatic fluid to the inner ear. Fishes with 

 this structure are identified as the Ostariophyses. In the 

 clupeoid fishes, paired anterior diverticula extend forward 

 from the swim bladder to contact the inner ear directly 

 (Figure 9-32). 



The endolymphatic duct is a short closed tube in teleosts. 

 It is missing in Satmo and Lampanyclus. The latter genus is 

 peculiar in having the sacculus (and lagena) separated from 

 the rest of the middle ear — the macula neglecta is in the 

 floor of the utriculus. 



The lateral-line system 



The lateral-line system is an integral part of the acustico- 

 lateralis system which includes the ear. It consists of sensory 

 lines radiating over the head and body, pit organs, and the 

 ampullae of Lorenzini. The basic lines (Figure 13-25) are 

 as follows: one above the eye, the supraorbital; another be- 

 hind and below the eye, the infraorbital; one paralleling 

 the mandibular arch and having jugal and oral parts; one 

 passing down over the hyoid arch and on to the lower jaw 

 and having preopercular and mandibular segments; a 

 temporal division, which continues posteriorly as the main 

 lateral line, and a supraoccipital connective joining the 

 temporal lines. In addition to these, there are anterior, mid- 

 dle, and posterior pit lines on the top of the head. On the 

 body, there are dorsal and ventral body lines in some 

 fishes in addition to the lateral line. 



The sensory lines of living forms have organs called neuro- 

 masts or maculae formed of clumps of cells. The cells of 

 a neuromast have hair-like projections enclosed in a cupola of 

 gelatinous material. Movement of the cupola, acting through 

 the hairs, stimulates the cells. These cells, in diflferent areas, 

 are innervated by the seventh, ninth, and tenth cranial nerves. 

 Whether these sensory structures are on the surface, in pits, 

 or in a canal does not seem to be important except as a func- 

 tional modification. 



This system functions in the detection and location of 

 disturbances in the water. Such a sense aids in finding food, 

 in social behavior, in avoiding enemies, and in echo lo- 

 cation. It functions also as an accessory rheotactic sense 

 (orientation in flowing water). In actinopterygians this 

 system is best developed in free-swimming forms, those liv- 

 ing in rapidly flowing water, or surf forms (Figure 13-30). 

 Having the organs located in closed canals is assumed to 

 represent a high development, while surface organs are 

 presumed to precede loss of this system. 



The morphology of this system will be reviewed begin- 

 ning with the agnaths. 



Agnafh This system in fossil agnaths (Figure 5-28 to 5-32) 

 appears to involve a "mucous-canal" reticulum as well as 

 sensory canals. The system is assumed to be one of epidermal 

 canals forming a reticular pattern over the head and trunk. 



410 • THE NERVOUS SYSTEM 



