SCALES. 



[ 563 ] 



SCALES. 



reous matter which is left after incinerating 

 the scales, retaining the original form of the 

 bodies (PI. 17. fig. 21). 



In the scales of some fishes, particularly 

 those of extinct genera and species, lacunae 

 and canahculi resembling those of bone 

 (PI. 17. fig. 1 c), with Haversian canals, 

 are met with. A vitreous or enamel-like 

 layer, having the structure of dentine, is also 

 met with in the form of an external coating. 

 The structm-e of the spines or spine-like 

 scales of the skate is curious. The larger of 

 them, consist of a button-like base, sur- 

 mounted by a sharp process (PI. 17. fig- 3). 

 The outer and lower part of the base is 

 opake-white, and consists of an imperfectly 

 fibrous tissue with large areolae (PI. I/. 

 fig. 37). The spine is hollow, the cavity 

 being continuous with that of a rounded 

 bodv, partly immersed in the white substance 

 (PI." 17. fig. 3 a). The cavity is filled with 

 a pulp, consisting of lax areolar tissue with 

 minute cells ; whilst its walls are composed 

 of a hard substance traversed by branched 

 canals resembling those of dentine (PI. 17. 

 fig. 4). The substance of the smaller spines 

 (PI. 17. fig. 2) exhibits the same dentinous 

 structure (fig. 5). 



PI. 17. fig. 10 represents one of a longitu- 

 dinal row of scales extending along the 

 middle of the side of the body of most fishes, 

 and traversed by a tube («), formerly sup- 

 posed to give exit to the mucous secretion 

 of the surface, which view has lately been 

 thrown into doubt. The tubes are visible to 

 the naked eye, and produce the lateral line 

 as it is called. 



The scales of fishes contain a large amount 

 of inorganic matter, composed principally of 

 phosphate of lime, but mixed with the car- 

 bonate. The organic basis consists of a 

 cartilaginous substance. 



Some years since, M. Agassiz founded a 

 classification of fishes upon the structure of 

 the scales, having found that with diff^erences 

 in the scales, other great and important 

 distinctions were in harmony. The system 

 has been found of eminent service to the 

 geologist; although later researches have 

 shown that scales presenting the character- 

 istics of those belonging to fishes of difi^erent 

 orders in this system, have been found upon 

 the same fish. 



The arrangement was as follows : — 

 '*Scales enamelled. 



Ord. 1. Ganoid fishes. Those the skin 

 of which is regularly covered with angular 

 thick scales, composed internally of bone, | 



and externally of enamel. Most of the species 

 are fossil, the sturgeon and bony pike being 

 recent. 



Ord. 2. Placoid fishes. Skin covered irre- 

 gularly with large or small plates or points 

 of enamel. Includes all the cartilaginous 

 fishes of Cuvier, except the sturgeon; as 

 examples may be mentioned the sharks and 

 rays. Many are fossil. 

 Scales not enamelled. 

 Ord. 3. Ctenoid fishes. Scales horny or 

 bony, serrated or spinous at the posterior 

 margin. Contains the perch and many other 

 existing species, but few fossil. 



Ord. 4. Cycloid fishes. Scales smooth, 

 horny or bony, entu-e at the posterior mar- 

 gin ; as the salmon, herring, roach, and most 

 of our edible and freshwater fishes. 



Most of the fossil fishes belong to the first 

 two orders ; and most of the recent to the 

 third and fourth. 



BiBL. Agassiz, Rech. sur les poissonsfos- 

 siles, Ann. d. Sc. nat. 2 ser. 14 ; Mandl, 

 Ann. d. Sc. nat. 2 ser. xi. xii. xiii. &xiv.; 

 Heusinger, Histolog. ii. 226 ; Stannius, 

 Vergl. Anat.; Peters, ilfiiWer's Archiv, 1841. 

 ecix.; Miiller, Wiegmann's Archiv, 1843.298; 

 Reade, Ann. Nat. Hist. 1838. ii. 191 ; Vogt, 

 Zoolog. Brief e, ii.; Williamson, FJiil. Trans. 

 1849. 435. 



SCALES, OF Insects. — The fine dust 

 which adheres so readily to the fingers on 

 handling a butterfly or moth, consists of a 

 number of microscopic flattened bodies, 

 called scales or feathers, and upon which 

 the beautiful colours and opacity of the 

 wings depend, the membranous wing itself 

 being transparent and colourless. 



These scales have always been favourite 

 microscopic objects, both on account of the 

 beauty and variety of their forms, and the 

 curious markings found upon them. The 

 manner in which they are attached is best 

 examined in the wing of a butterfly. Each 

 has a naiTOw portion at its base, forming a 

 pedicle or stalk. The stalks are implanted 

 into small and short tubes or cups (PI. 27. 

 fig. 23 b), denominated the squamuhferous 

 tubes, the orifices of which are directed 

 backwards. Around the points of attach- 

 ment of the cups to the wings, the surface 

 exhibits a number of irregularly radiating 

 rvigaj or folds of the upper membrane of the 

 wing (PL 27. fig. 26). The cups are arranged 

 in more or less regular transverse rows. 



The scales are variable in form, both in 

 diff^erent insects and in difi^erent parts of the 

 same insect, being oval, oblong, cordate, 



2o2 



