FINS INTEGUMENT. 131 



bony, pointed spines the so-called spine-rays (Acanthopieri) or by 

 soft jointed rays (Mcdaoopteri). The caudal fin is as a rule composed 

 of a part of the dorsal and a part of the ventral fin-fold, but it varies 

 much in its form. When the dorsal and ventral lobes are symmetrical 

 the caudal fin is said to be homocercal ; when the ventral lobe is the 

 larger, in which case the caudal part of the vertebral column is usually 

 bent dorsalwards, the caudal fin is said to be heterocercal. It some- 

 times happens, however, that while the caudal fin is externally homo- 

 cercal the axial skeleton is bent dorsalwards so that the fin is 

 internally heterocercal. 



The paired pectoral and pelvic fins correspond to the anterior and 

 posterior limbs of other Vertebrates. The former are attached to the 

 head immediately behind the gills by means of an arched shoulder- 

 girdle, while the two pelvic fins are approached to the middle line 

 and placed further back, usually on the abdomen (ventral fins) ; 

 sometimes, however, they lie between the pectoral fins (thoracic fins), 

 and more rarely in front of the latter on the throat (jugular fins). 



The integument of fishes is seldom completely naked (Cyclostomi). 

 As a rule scales ossifications of dermal papillae, which are completely 

 covered by epidermis are embedded in it. The scales are often 

 so small that they are hidden beneath the skin and seem to be 

 completely absent (Eels). As a rule, however, they are present as firm, 

 more or less flexible plates, which are covered with a number of 

 concentric lines and radial striations and lie on one another like 

 slates on a roof. Scales may be distinguished according to the 

 structure of their free edges as cycloid scales with smooth edges, and 

 ctenoid scales with serrated edges. Scales, which overlap but little 

 and are generally rhomboidal, more rarely cycloidal in shape, and 

 have an outer layer of enamel, are called ganoid scales, while the 

 term placoid scale is applied to the small bony granules (composed 

 of enamel and dentine) of different shapes, which lend to the surface 

 of the skin the appearance of shagreen (these are the primitive form 

 of teeth). Agassiz divided the Pishes according to the shape of their 

 scales into Cycloids, Ctenoids. Ganoids, and Placoids. 



In the skin there are peculiar cutaneous canals communicating 

 with the exterior by lateral rows of pores. These are called the 

 lateral lines and were considered to be slime-secreting glands till 

 Leydig* discovered that they contain a sense organ. 



* Compare Leydig, " Ueber das Organ eines sechsten Sinnes." Dresden, 1868 

 Fr. E. Schulze, "Ueber die Sinnesorgane der Seitenlinie bei Fischen uncl 

 Amphibien." Arch, fur mikrosk. Anatomie, Tom. VI., 1870. 



