PISCES. 



411 



tubule is dilated and either simple or sacculated, and contains sense-cells 

 in the lining epithelium. The ampullae are filled with gelatinous mucus. 

 End-buds are as a rule plentifully scattered over the head and body in 

 Teleostei, but they may be wanting entirely, e. g. in Mugil and Esox, where 

 scattered nerve-eminences occur. They have only been detected in Torpedo 

 among Elasmobranchii. But in all Fish with few exceptions, where they are 

 replaced by nerve-eminences, they occur in the mouth and on the branchial 

 arches. The sense-cells of the buds differ from those of nerve-eminences 

 in extending from the surface to the base of the epidermis. End-buds are 

 also generally prominent. 



Curious 'eye-like' organs are imbedded in the skin of certain pelagic 

 and deep-sea Teleostei (Scopelus, Chauliodus, &c.). According to peculiarities 

 of structure these organs are divisible into ' eye-like organs proper/ * pearl- 

 like/ and 'phosphorescent' organs. They all agree in possessing an 

 iridescent layer resembling a tapetum, and like it reflecting light. The 

 mucus coating the surface of the body is often phosphorescent, and some 

 Fish possess phosphorescent organs, e. g. an appendage beneath the head 

 in Eustomias. 



The exoskeleton is rarely absent, e.g. in many Eels and all electric 

 Fish ; it is sometimes rudimentary. The primitive form occurs in Elasmo- 

 branchii in the shape of dermal teeth ( = placoid scales), similar in structure 

 and development to oral teeth and borne upon a plate of bone formed 

 in the cutis 1 . The Siluroidei among Teleostei have dermal teeth also, 

 moveably articulated, however, to their bony supports, and these supports 

 often fuse into large plates. In other Fish teeth tend to disappear, and 

 the supporting bone constitutes the scales, scutes, spines, or shields of 

 the exoskeleton. Teeth, however, are found on the skin between the 

 mandibles of Lepidosteus, and as minute spines on the edges of its scales. 

 The enamel, however, is homogeneous, the dentine tubular and a pulp- 

 cavity absent. Minute denticles of non-tubular dentine are also found 

 on the outer surface of the scales of Protopterus (Dipnoi]. The spines 

 projecting through the epidermis from the scales and scutes of Acipenser 

 are homogeneous structures. The scales of Lepidosteus and Polypterus 

 among living (as of many extinct) Ganoidei are covered with a layer of 

 homogeneous enamel. They are lodged, as are the scales of Teleostei and 

 Dipnoi, in sacs of the cutis. The scales thin out at their edges and vary 

 much in size. They are cycloid or ctenoid according as their projecting 

 margin is entire or denticulated. Among Plectognathi (Teleostei} the 

 Ostraciontidae are covered with a mail of large polygonal plates ; and the 



1 Pristis, the Saw-fish, one of the Rays, has a long flat prae-nasal cartilage. A linear series of 

 teeth, similar in structure to the oral teeth of Myliobatis, another Ray, are implanted in sockets along 

 the edge of this rostrum. They grow from persistent pulps. The Saw-fish is said to glide rapidly 

 by a whale and to rip it with these weapons. 



