36 THE ELEMENTS OF STRUCTUKE. 



is when we compare these degenerate forms with their 

 ancestral ideal, {c) But among " rudimentary organs " we 

 also include structures somewhat different, e.g., the gill 

 clefts which persist in embryonic reptiles, birds, and 

 mammals, though they serve no obvious purpose, or the 

 embryonic teeth of whalebone whales. These are " vestigial 

 structures" traces of ancestral history, and intelligible on no 

 other theory. The gill clefts are used for respiration in all 

 vertebrates below reptiles ; the ancestors of whalebone 

 whales doubtless had functional teeth. In regard to these 

 persistent vestigial structures, it must also be recognised 

 that we are not warranted in calling them useless. Though 

 they themselves are not functional, they may sometimes be, 

 as Kleinenberg suggests, necessary for the growth of other 

 structures which are useful. 



Classification of Organs. — We may arrange the various parts of the 

 body physiologically, according to their share in the life. Thus, some 

 parts have most to do with the e.xlernal relations of the animals ; such 

 are locomotor, prehensile, food-receiving, protective, aggressive, and 

 copiilatory organs. Of internal parts, the skeletal structures are passive ; 

 the nervous, muscular, and glandular parts are active. The reproductive 

 organs are distinct from all the rest. They are often called " gonads," 

 and should never be called glands. For by a gland we mean an organ 

 which secretes, an organ whose cells produce and liberate some definite 

 chemical substance, such as a digestive ferment. Whereas the gonads 

 are organs in which certain cells, kept apart from the specialisation 

 characteristic of most of the " body cells " or " somatic " cells, are 

 multiplied, and eventually liberated. 



Another classification of organs is embryological. i.e., according to the 

 embryonic layer from which the various parts arise. Thus, the outer 

 layer of the embryo (the ectoderm or epiblast) forms in the adult (l) 

 the outer skin or epidermis ; (2) the nervous system ; (3) much at least 

 of the sense organs : the inner layer of the embryo (the endoderm or 

 hypoblast) forms at least an important part (the "mid gut "') of the food 

 canal, and the basis of outgrowths (lungs, liver, pancreas, &c.) which 

 may arise therefrom, and also the notochord of Vertebrates : the middle 

 layer of the embryo (the mesoderm or mesoblast) forms skeleton, con- 

 nective swathings, muscle, cS:c. 



It is important to adopt some order of description. It is obviously 

 prejudicial to the success of your work and to the health of your brains, 

 to describe an animal in any order that occurs to you, to skip from food- 

 canal to kidney, or from heart to reproductive organs. Therefore, in my 

 descriptions I shall follow, almost consistently, this order of treatment :— 

 Mode of life, form, external appendages, skin, skeleton, muscle, nervous 

 system, sense organs, food canal, body cavity, vascular svstem, respira- 

 tory system, excretory system, reproductive system, development. 



