38 THE ELEMENTS OF STRUCTURE. 



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 external relations of the animals ; such 

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

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

 the nervous, muscular, and glandular parts are active. The repro- 

 ductive 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. 



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 — ( I ) 

 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, etc.) which 

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

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

 connective swathings, muscle, etc. 



It is important to adopt some order of description, and in the descrip- 

 tions of animals given in this book, we 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 system, respiratory system, excretory system, reproductive 

 system, development. 



III. Tissues. — Zoological anatomists, of whom Cuvier 

 may be taken as a type, analyse animals into their com- 

 ponent organs, and discover the homologies between one 

 animal and another. But as early as 1801, Bichat had 

 published his " Anatomie gdnerale," in which he carried the 

 analysis further, showing that the organs were composed of 

 tissues, contractile, nervous, glandular, etc. In 1838-39, 

 Schwann and Schleiden formulated the " cell theory," in 



