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CUTANEOUS SYSTEM. 



of down consisting of minute filaments collected in 

 tufts, which are implanted in a bulb or follicle which 

 contains the sheath of the feather. When the sheath 

 protrudes through the skin it has the appearance of a 

 thorn or little cone, within which may be observed 

 the medulla or that central gelatinous cord which 

 may be considered to form the different parts of the 

 feather. Upon its protrusion the tufts just described 

 generally fall off, excepting in the eagle, vulture, and 

 other rapacious birds, to the feathers of which it re- 

 mains attached for a considerable time resembling 

 fine down. From the medulla or pith the barbs are 

 first discovered proceeding from it in very delicate 

 and thin filaments ; then the shaft or stalk of the 

 feather makes its appearance, to which they soon 

 become attached ; the whole being enclosed within 

 the sheath, the barrel end of which is filled with a 

 glairy fluid. In proportion as this fluid becomes 

 converted into the barbs and shaft, the central cord 

 withers and corrugates. The sheath is at length 

 burst open, and the medulla appears encircled by the 

 barbs and shaft. These parts being formed, the re- 

 maining part of the medulla becomes enveloped with 

 the matter of the quill which is a continuation from 

 the shaft. The quill therefore is the last part formed, 

 and continues to increase in length for some time 

 after the full growth of the barbs and shaft. When 

 feathers have reached their full growth they become 

 dry, and only the tuber or medulla within it continues 

 to absorb moisture at its termination or bulb. It 

 gradually, however, ceases to receive this nourish- 

 ment ; its attachment to the skin then becomes 

 loosened ; and the feather is cast, and again in the 

 manner described renewed. This periodical change 

 of plumage is termed moulting-, during which period 

 inrds generally become sickly and much debilitated. 

 Immediately below the feathers of aquatic birds, in 

 the region especially of the chest, other small and 

 very delicate feathers are found which are called 

 down, which serves to protect the body from the 

 action of the water, and which, not being good con- 

 ductors of heat, likewise preserve the temperature of 

 the body. The down of the eider duck is esteemed 

 the most, valuable. These birds pluck it from their 

 breasts to line their nests with it. So elastic is it 

 that a quantity weighing only three quarters of an 

 ounce will spread over a larger surface than the 

 crown of the greatest hat. That found in the nest, 

 which is termed live down, is much more elastic than 

 that plucked from the dead bird, and is more highly 

 esteemed. 



The colour of the plumage of birds varies exceed- 

 ingly, being in many species, especially in tropical 

 climates, of dazzling brilliancy. The iridescence of 

 the plumage is referable to the same cause as the 

 colours of very thin plates, which has been explained 

 in the article COLOUR. It is, however, remarkable 

 that the colour of the plumage of certain birds under- 

 goes a change at different seasons of the year. 

 Among aquatic birds, the black guillemot ( Una Grylle] 

 is of a sooty black in summer, with a patch of white 

 on each wing ; as winter approaches, the black gra- 

 dually disappears, until the plumage becomes white, 

 mottled with ash-coloured spots. Again, the ptarmi- 

 gan or mountain grouse (Tetrao lagopus) has an ash 

 coloured plumage, which, however, changes in winter 

 to a snow-white ; a providential change, says Mon- 

 tagu, for, were the surface of the ground not 

 consonant with their colour, few would escape the 



searching eye of the falcon or the eagle in the lofty 

 and exposed situations they are found to inhabit. It 

 has therefore been wisely ordained that the young 

 first appear, like their parents, in a mottled plumage, 

 similar to the lichen-covered rocks they frequent, 

 and continue in this dress till the approacii of winter, 

 when old and young become equally white as the 

 surrounding snow. In the Scottish Highlands this 

 change commences about the month of September ; 

 large patches of white appear on the feathers of the 

 wings and back ; these extend, and in the month of 

 January the bird is of a pure white. The return of 

 colour commences by the appearance of black spots, 

 which enlarge ; they become of a brownish hue, 

 until the colour and the summer plumage is com- 

 pletely resumed. This change of colour is, however, 

 sometimes limited to a certain part of the body. 

 Thus the cheeks and throat of the little auk (Ak-a 

 alle) are black in summer, and become of a dirty 

 white in winter ; the head of the black-headed gull 

 (Larus ridibundus}, as the name indicates, is black in 

 winter, and becomes white in summer. 



SHELLS and CRUSTS are a part of the cutaneous 

 system of the Crustacea, besides which they occur to 

 several of the annelides, as the serpulaand dcntalium. 

 They consist simply of layers of earthy matter inter- 

 posed between membranes of animal matter. This 

 deposition takes place between the epidermis or 

 cuticle and the cutis vera or true skin. It occupies 

 the place of the rete mucosum above described. 

 Crusts differ from shells only in being more brittle, in 

 consequence of being composed of a greater quantity 

 of earthy matter. In shells the carbonate of lime 

 predominates ; in crusts there is also a considerable 

 portion of phosphate of lime. The size, texture, shape, 

 and colour of shells, display an infinite variety. The 

 shield of the snail is semi-transparent and elastic, the 

 shell of the volutes opaque and hard as marble. River 

 shells are not so diversified in form and colour as 

 land and sea shells ; but so numerous and varied 

 are the characters of shells, that their study and 

 classification constitutes, under the term CONCHOLOGY, 

 a complete department of natural history. 



The crusts of the Crustacea like the hair of quad- 

 rupeds, or feathers of birds, are cast periodically, and 

 when injured or destroyed are easily renewed. When 

 this change occurs in the lobster, his shell grows soft, 

 the sutures open, and the claws burst at the joints. 

 Thus loosened, the animal, by a trembling motion, or 

 sort of spasm casts off his case, and then retires in a 

 state of nakedness to the hollow of rocks. The re- 

 leased body then makes a sudden growth, and in about 

 eight and forty hours a fresh concretion of humour is 

 deposited over the whole surface of the body, which 

 soon hardens into a crust, which is adapted in every 

 part to the increased size of the animal. This change 

 takes place annually. Besides in the Crustacea, pro- 

 perly so called, crusts occur in certain insects, echino- 

 dermata or sea urchins, and starfish. 



Lastly. SCALES are the cutaneous appendages or 

 covering proper to fish ; they are found also in every 

 part of the body of certain reptiles, and also upon the 

 feet of birds. They are composed of the same matter 

 as the cuticle which extends over them ; and they 

 vary considerably in size, form, colour, and relative 

 position. Such are the appendages, which constitute 

 a part of the cutaneous system of the different classes 

 of animals which inhabit the earth, the air, and the 

 waters ; and which afford a protection adapted in the 



