INTEGUMENT 



19 



cells commonly present may be mentioned the granular cells in 

 the Lamprey (the nature of which is not understood), and in 

 Myxinoids the numerous slime-sacs, formed as invaginations of the 

 ep'iderm and containing peculiar thread- cells. 



Special aggregations of gland-cells occur in relation with the 

 copulatory organs or claspers of male Elasmobranchs (glandulce 

 pterygopodii), and on the operculum and dorsal fin-rays of certain 

 Acanthopteri (e.g. Trachinus, Thalassophryne, Synanceia), in 

 which latter they constitute a poison-apparatus serving for offence 

 or defence, and consist of modified epidermic cells enclosed in 

 grooves of the spines of the operculum and dorsal fins. Most 



Scales. 



Mucus cells. 



Pocket enclosing 

 scale. 



Epiderm. 



Muscles. 



FIG. 13. LONGITUDINAL SECTION OF SKIN OF YOUNG TROUT (15 CM. LONG), 



FROM THE TAIL. 



of these poison-fishes are inhabitants of the temperate and 

 warmer seas: in fresh-water forms (e.g. Perca, Cottus), the 

 apparatus has apparently undergone partial or total degeneration. 

 Poison-organs are also said to occur in a number of other Teleosts 

 (e.g. in connection with the dorsal and pectoral spines of many 

 Siluridae) and in certain Elasmobranchs ; a closer examination will 

 probably prove their existence in many other Fishes. 



Phosphorescent organs, formerly known from their appearance 

 as " accessory eyes/' occur on various parts of the head, body, and 

 tail of several families of deep-sea Teleosts (e.g. StoiniatidaB, 

 Halosaurida3, Anomalopidse), and in certain species of Elasmo- 

 branchs belonging to the family Spinacidse. Their arrangement, 

 distribution, and structure is very varied in different forms. The 

 luminous part consists of gland-cells, supplied by the trigeminal, 



c 2 



