46 THE ELEMENTS OF STRUCTURE 



similarly spoilt in the making. They illustrate arrested 

 development, (b) Some animals lose, in the course of their 

 life, many of the prominent characteristics of their larval 

 Hfe ; thus parasitic crustaceans at first free-living, and sessile 

 sea-squirts at first free-swimming, always undergo degenera- 

 tion, which can be seen in each lifetime, {c) But the little 

 kiwi of New Zealand, with mere apologies for wings, 

 and many cave fishes and cave crustaceans with slight 

 hints of eyes, illustrate degeneration, which has taken such 

 a hold of the animals that the young stages also are degener- 

 ate. The retrogression cannot be seen in each lifetime, 

 evident as it is when we compare these degenerate forms with 

 probable ancestors, (d) But among '* rudimentary organs " 

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

 clefts which persist in embryonic reptiles, birds, and 

 mammals, though most of them serve no obvious purpose, 

 or the embryonic teeth of whalebone whales. These are 

 " vestigial structures,^' traces of ancestral history, and in- 

 telligible 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. 



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

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

 parts liave 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 conveniently 

 called " gonads," which is a better term than reproductive glands. 

 For by a gland we mean an organ which secretes, whose cells produce 

 and liberate some definite chemical substance, such as a digestive 

 ferment ; whereas the gonads are organs where there is periodic multi- 

 plication of certain cells, kept apart from the specialisation character- 

 istic of most of the " body cells " or " somatic " cells. It is true, 

 however, that an accessory glandular function is often associated with 

 the gonads. 



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, lining of body-cavity, etc. 



