SENSE-ORGANS OF THE INTEGUMENT 191 



a level with, the epidermis, 1 but in Fishes they may in post-em- 

 bryonic time become enclosed in depressions or complete canals, - 

 which are formed either by the epider- 

 mis only, or, as is more usually the case, 

 by the scales and bones of the head, and 

 which open externally. The organs are 

 thus protected. 



These sensory organs are situated 

 characteristically along certain tracts, 

 the position of which is very constant : 

 in the head, supra-orbital, infra-orbital, 



and liyomandibular tracts can be recog- 



, J , , 7 . f FIG. loO. TRANSVERSE SEC- 



nised, and a lateral line, (or several TIOX OF A F REELY p RO - 



Proteus and all Amphibian larvae) ex- JECTING SEGMENTAL SENSE- 



tends along the sides of the body to the ORGAN. 



caudal fin (Figs. 152 and 153). They The cuticular tube and the 



are thus often spoken of as segmental ^Zf SnTet c"! 



sensory organs or organs of the lateral central (sensory) cells ;MZ, 



line? The portions -lying in the region MZ 1 , peripheral cells. 



of the head are innervated by the lateral 



line branches of the facial, glossopharyngeal, and vagus (see note 



on p. 185). 



Freely projecting nerve-eminences are not present in Rays and 

 Ganoids, and are only of minor importance in Sharks. In all 

 these Fishes the integumentary sense-organs are more or less deeply 

 situated, being enclosed in complete or incomplete canals arising 

 as proliferations of the epidermis extending into the dermis, and 

 becoming greatly branched. 



The so-called Savi's vesicles of Torpedo, the ''nerve sacs" of 

 Ganoids, and the ampullae of Elasmobranchs, correspond to modified 

 nerve-eminences. They are all limited in their distribution to the 

 head and anterior portion of the trunk, being most numerous on the 

 snout : they arise from thickenings of the epidermis which later 

 become invaginated and in which a sensory epithelium is differ- 

 entiated. In Ganoids these organs retain a simple sac-like form, 

 and in Torpedo they become completely separated off from the 

 epidermis, while in other Elasmobranchs they are tabular, each 

 tube giving rise to one or more swellings or ampullae, separated 



1 At the time when an Amphibian undergoes metamorphosis and gives up its 

 aquatic habits, these sensory organs sink downwards into the deeper layer of the 

 skin, and, as the epidermis grows together over them, they become shut off from 

 the exterior and reduced, and may finally disappear. (Anura and certain Caduci- 

 branchiata. ) In other Urodeles they may, in some cases, be retained throughout 

 life, and are said to come to the surface when the animal returns to tne water 

 during the breeding season ; but, more usually, new organs then become 

 developed. 



2 This is also the case on the head in Dipnoans. 



3 In the Dipnoi they are not limited to the lateral line, and in Marsipobranchii 

 they have no regular arrangement and are not numerous, although a lateral 

 branch of the vagus is present. 



