358 



TELEOSTEI 



surface more or less evenly (Fig. 344). Occasionally they occur 

 near the eyes, and so placed as to direct the light inwards. Some- 

 times they appear to be directly derived from the lateral line on the 

 body, and may be lodged in a specially enlarged series of scales 

 (Fig. 417). The interesting suggestion has been made that, since 

 they give out light of various colours, these luminous organs fulfil 

 the same function among deep-sea fish living in the dark as do pig- 

 ments among animals in daylight (Brauer [43-44], Lendenfeld [280]). 



The lateral line is often excessively developed on the head, 

 large mucus cavities being excavated in the bones (Scorpenidae). 

 On the trunk the canal may be obliterated so that the organs lie 

 in the skin (p. 222). 



The Teleostei never have more than four complete gills, and 



adf. 



A. 



FIG. 344. 



A, Scopelius engrcmlis, Gthr. B, Polylpnus spinosus, Gthr. (After Giinther.) ./, anal lin ; 

 ad/, adipose fin ; d.f, dorsal fin ; p.f, pelvic' fin ; ph, phosphorescent organ ; pt.f, pectoral tin. 



often less in the specialised groups. Frequently there is a pseudo- 

 branch of variable structure and doubtful morphological signi- 

 ficance. It may be well developed and in the form of gill-lamellae, 

 as in the Pleuronectidae ; or it may be more or less completely 

 hidden below the skin or even deeply sunk in the tissues. In many 

 cases it is said to be altogether absent. The blood -supply is 

 derived from the efferent system of vessels, and usually from the 

 hyoidean artery (second vascular arch) ; while it leaves the pseudo- 

 branch by the ophthalmic artery (Fig. 72). It has not yet been 

 determined for certain whether this pseudobranch is derived from 

 the mandibular or the hyoidean gill ; the latter seems to be its 

 place of origin in many cases, but in others it may develop from 

 both sources. (Allis [15], Cole and Johnstone [84], Dohrn [115], 

 Maurer [294-95]). 



