York, and some of these have been suitable 



for sectioning and microscopic examination. 



Invasion of the Land by Arthropods. The 



sea, of course, was the "primordial mother of 

 life." In fact, the fossil record shows that no 

 plants or animals succeeded in colonizing 

 any land area until Silurian times. The rec- 

 ord also shows that the first animals to dis- 

 play definite adaptations to land conditions 

 were scorpionlike Arachnida that began to 

 occupy the continental edges some 400 mil- 

 lion years ago. In other words, the first land 

 arthropods anticipated the first land verte- 

 brates ('Amphibia) by more than 60 million 

 years. Furthermore, the Arthropoda are the 

 only invertebrate group in which large num- 

 bers and a great diversitv of terrestrial species 

 have been evolved. A few Annelida, such as 

 earthworms, and a few Mollusca, such as 

 snails, are terrestrial, especially in damp en- 

 vironments; but among living Arthropoda, 

 only the Crustacea are aquatic. In fact, a 

 great majority of centipedes, millipedes, 

 arachnids, and, above all, insects are very 

 well-adapted terrestrial animals. 



Arthropod Adaptations to Land Conditions. 

 Xo organism can live if it loses too much 

 water, and the exposure of a land animal to 

 the drying effects of the atmosphere repre- 

 sents an ever-present hazard. Arthropods, 

 however, are covered most completely by a 

 chitinous exoskeleton that controls the loss 

 of water from the soft internal living parts 

 of the body. In fact, the chitinous, wax-laden 

 exoskeleton of the arthropod stands on a par 

 of efficiency with the integumentary coverings 

 (skin, with scales, feathers, or hair) of various 

 vertebrate animals in protecting the body, 

 not only from desiccation, but also from vari- 

 ous other injurious external factors. More- 

 over, a strong exoskeleton enables the animal 

 to cope with gravitational force, which has 

 a greater effect upon land where the buoy- 

 ancy of the aquatic environment is lacking. 



Respiration in land animals also presents 

 a special problem. The respiratory surface, 

 across which oxygen and carbon dioxide 

 enter and leave the blood, cannot function 



The Animal Kingdom - 657 



properly if it becomes too dry. The book 

 lungs and tracheal tubes of terrestrial arthro- 

 pods, as well as the true lungs of terrestrial 

 vertebrates, represent a deep insinking of the 

 respiratory surfaces to a safely recessed posi- 

 tion, where desiccation can scarcelv occur. 

 The book lung, essentially, consists of a num- 

 ber (usuallv around twenty) of thin, richly 

 vascularized, leaflike plates, arranged like 

 slightly separated pages in a book, and occu- 

 pying an inpocketed chamber, usually in the 

 anterior region of the abdomen. The tra- 

 cheal system, on the other hand, consists of 

 an elaborately branching system of air-bear- 

 ing tubes. The finer (microscopic) tracheal 

 branches permeate the tissues, carrying air 

 to the immediate vicinitv of the individual 

 cells. The larger trunk-line tracheae com- 

 municate with the outside air through paired 

 openings, the spiracles, which are present in 

 each typical segment of the thorax and abdo- 

 men. Each tube of the system is kept from 

 collapsing by spirally wound chitinous 

 threads which reinforce the wall. A continu- 

 ous ventilation of the tracheal system (or of 

 the book-lung chamber) is effected by rhyth- 

 mic expansions and contractions of the ab- 

 domen and thus new air keeps reaching the 

 finer branches, where the cells take in oxv- 

 gen and give off carbon dioxide. The tra- 

 cheal system, particularly, is well adapted to 

 meet the respiratory requirements of insects. 

 The primitive circulatory system of these 

 very active animals is not designed for carry- 

 ing large amounts of oxygen, which are 

 needed especially during periods when flight 

 must be sustained. 



A few terrestrial arthropods revert to the 

 ancestral habit of depositing their eggs in an 

 aquatic environment; but most lay eggs on 

 or in the soil, or in other places more or 

 less exposed to the drying effects of the at- 

 mosphere. Such eggs, typically, are covered 

 by a tough moisture-proof chitinous shell, or 

 integument, which represents an adaptation 

 to terrestrial development. Many such ani- 

 mals, especially insects, resort to copulation. 

 Thus fertilization occurs in the deep recesses 



