DERMAL SKELETON 



211 



their scales. The first have only 'placoid' scales (denticles, 

 see p. 119). The second have the flat rhombic shiny scales 

 mentioned above. The third have thin rounded scales. Lastly, 

 the Ctenoidei have similar thin scales, but provided with spiny 



Fio. 183. 



Ltin-isriin rutilus. Left-side view, showing the myotomes, two of which are marked in black. 

 E, longitudinal transverse septum ; S, lateral line. (After A. Hase.) 



projections. Kecent work tends to show that the ' cycloid ' and 

 ' ctenoid ' scales are closely allied forms derived from the more 

 primitive ' ganoid ' scale, and that they all three differ very 



Fio. 184. 



Dermal lin-ray of Polypterus bichir, Geoflr., enlarged. A, surface view of a few segments. 

 H. longitudinal section of the same, ct, connecting ligament ; d, denticle ; cp, epidermis ; si; 

 bony plate or segment of lepidotrich. 



fundamentally from the 'placoid' type (Williamson [496], Hertwig 

 [212], Klaatsch [264], A. S. Woodward [505]). 



The true scale (as distinguished from the placoid denticle) of 

 the Osteichthyes sinks deep into the connective tissues, it is rarely 

 shed, and generally grows throughout life by the addition of 

 successive layers of bony substance. Scales differ, however, very 



