II. Organs. 1. The Shm. 2. Skeleton. 15 



tave areas of sticky skin by which they can adhere to other bodies (Amphibiaii 

 larvae), others can hold fast by simple adhesion,* applying a surface which is 

 smooth and damp, but not sticky, to the foreign object (Tree-frog) ; others again 

 can fix themselves by means of hooks, &c. 



The oolom- of the skin depends, in many oases, upon cells containing pigment 

 granules, present either in the dei-mis or the epidermis (or in both). The 

 pigment varies in chemical composition. Sometimes the blood in the dermal 

 blood-vessels shows thi-ough the skin (cocks' combs). The colour of the skin may,, 

 however, depend on totally different circmnstanoes : for instance, interference 

 colours* are known, which are due to the peculiar stnicture of the skin,. 

 e.g., the stratification of cuticle or dermis ; the well-known metallic colours are 

 often caused thus. The skin, or its derivatives, is sometimes white (e.g.,. 

 hair of Mammalia) ; this occasionally depends upon the presence of tiny air 

 vesicles. 



Moulting is frequent in animals ; the outer layer of the 

 epidermis, either the cuticle (Insecta, Crustacea), or the stratum 

 corneum (Vertebrata) becomes loosened from the rest of the skin,, 

 and is thrown off all at once, or more rarely in pieces. Such a 

 moult is always accompanied by a new formation of cuticle or of 

 stratum corneum, which has always begun before the animal casts 

 the old one. 



Mucous membrane is the name given to the skin-like .lining of the 

 cavities of the body which open directly on to the outer surface {e.g. the 

 digestive tract) ; it consists of an epithelium supported by a layer of connective- 

 tissue, corresponding with the epidermis and dermis of the skin. 



2. Skeleton. 



The protective structures found upon, or the calcifications and 

 ossifications present in, the skin, which have just been referred to,, 

 frequently attain a considerable thickness, hardness, and coherence, 

 and then form the supporting organ of the body, the skeleton,, 

 under which name all the hard parts are included. The skeletal 

 elements of the skin, the exoskeleton, owes its origin either- 

 to the epidermis, as, e.g., in the Lobster, where the well-developed 

 thick calcareous cuticle forms the supporting structure, or, as in the 

 Snail, whose shell is the secretion of certain epidermal cells; or to 

 the dermis, e.g. the numerous calcareous plates forming the shell of 

 the Sea-urchin, or the bony plates which compose the carapace of the 

 Turtle. In many animals, however, especially in the Vertebrata 

 there is also a firm supporting framework, the endoskeleton 

 which lies within the body and is quite independent of the skin : it 

 consists chiefly of cartilage and bone, and is often present in 

 addition to the exoskeleton, with some portions of which it may 

 be intimately connected (Tortoise). 



The exoskeleton comes to have, in most cases, a protective as well as a 

 supporting function, often indeed, this is its principal duty, e.g., MoUuscs and 



* See Text-book of Physics. 



