i STRUCTURE AND PHYSIOLOGY OF ANIMALS 31 



The sperms are developed by a succession of cell-divisions from 

 certain cells the primitive male cells similar in character to 

 immature ova. 



5. ORGANS. 



The chief systems of organs of an animal are the integumen- 

 tary, the skeletal, the muscular, the alimentary or digestive, the 

 vascular, the respiratory, the nervous, the excretory, and the repro- 

 ductive. 



The skin or integument consists in the majority of animals 

 of a cellular membrane the epidermis to which reference has 

 already been made, with, superficial to it, in many animals, a non- 

 cellular layer the cuticle, and below it usually a fibrous layer which 

 is known as the dermis. The epidermis may consist of a single 

 layer or may be stratified ; it is frequently ciliated, and some of 

 its cells frequently assume the form of unicellular glands. Modi- 

 fication of its superficial layers of cells gives rise frequently to the 

 formation of hard structures contributing to the development of 

 an exoskeleton (vide infra). 



The cuticle, when present, varies greatly in thickness and con- 

 sistency. Sometimes it is very thin and delicate ; in many 

 animals it becomes greatly thickened and hardened so as to form 

 a strong protecting crust, sometimes of a material termed chitin, 

 somewhat akin to horn in consistency, sometimes solidified by the 

 deposition of calcareous salts. The cuticle is to be looked upon as 

 a secretion from the cells of the epidermis; but the term is 

 frequently applied in the case of the higher animals in which a 

 cuticle in the strict sense of the term is absent either to a super- 

 ficial part of the epidermis, in which the cells have become altered 

 and horny, or to the whole of that layer. 



The layer or layers of the integument situated beneath the 

 epiderm consist of fibrous connective tissue and muscular fibres, 

 constituting, as mentioned above, the derm or dermis. 



The term skeleton or skeletal system is applied to a system 

 of hard parts, external or internal, which serves for the protection 

 and support of softer organs and often for the attachment of muscles. 

 This system of hard parts may be external, enclosing the soft 

 parts, or it may lie deep within the latter, covered by integument 

 and muscles : in the former case it is termed an exoskeleton or 

 external skeleton ; in the latter an endoskeleton or internal skeleton. 

 In many groups of animals both systems are developed. An 

 exoskeleton is formed by the thickening and hardening of a part 

 or the whole of one of the layers of the integument enumerated 

 above ; or more than one of these layers may take part in its 

 formation. In many invertebrate animals, such as Insects. 

 Crustaceans, and Molluscs, it is a greatly thickened and hardened 



