228 CLASSIFICATION OF VERTEBRATES. 



Under the heading Pisces are included all those forms, 

 except the lampreys and hags, commonly known as fishes. 

 The group is well marked off from all other groups of verte- 

 brates to-day, and the only questions of classification are those 

 pertaining to the relationships and arrangement of the various 

 groups composing the class. 



The skin usually contains abundant gland cells secreting 

 mucus, but multicellular glands are rare. The body is usually 

 covered with scales of dermal origin ; but these are occasionally 

 absent, as, for instance, in most electrical fishes. Some years 

 ago the scales were of importance in classification, and four 

 types were recognized : (i), placoid scales, occurring in the 

 elasmobranchs, in which there is a basal plate of dentine bearing 

 a central spine (p. 92) of dermal origin, the spine being tipped 

 with ectodermal enamel. This type of scale is regarded as the 

 most primitive, and as having given rise to teeth and dermal 

 bones (p. 164). (2), Ganoid scales ; large, bony (dermal) rhom- 

 boidal plates embedded in the skin, and frequently bearing on 

 their outer surface a layer of enamel (ganoin), likewise of dermal 

 origin. This type of scale is found in most ganoids, and was 

 especially characteristic of the early members of this group. 

 (3)' Cycloid scales, more horny in character, lacking in enamel, 

 and embedded in dermal pockets ; these have the outline 

 approximately circular. (4), Ctenoid scales differ from these 

 last in having small spines upon the posterior or free margin. 

 Occasionally scales may fuse to form large bony plates or 

 scutes. 



Median fins are always present, and, except in degenerate 

 forms, two pairs of paired fins as well. These are supported by 

 rays of dermal origin, and the paired fins also have cartilagi- 

 nous or bony basal supports (p. 177). In the young the median 

 fins are continuous around the tail, and this condition persists 

 in the adult of a few forms (e.g., eels) ; but usually it is inter- 

 rupted so that we may recognize fins on the back (dorsals), on 

 the tail (caudal), and on the ventral surface behind the vent 

 (anal). The shapes of these and the number of supporting 

 rays are of importance in the discrimination of species, etc., but 

 more important are some of the peculiarities of the caudal fin. 



