ZOOPHYTES 469 



membranes which line internal cavities and cover the external 

 surface, having in the latter case the name of epidermis; 2. Sup- 

 porting tissues, such as bone, gristle (cartilage), and connective 

 tissue which makes up tendons and ligaments, while it is also 

 found as a sort of supporting framework in almost all parts of 

 the body ; 3. Muscle, constituting the flesh or meat, and also 

 entering largely into the construction of the walls of various 

 internal organs, such as heart and stomach; 4. Nervous tissue, 

 constituting the essential part of the nervous system; and 5. 

 Blood and Lymph, which must be looked upon as liquid tissues, 

 serving as media of exchange. In a higher animal these various 

 tissues are highly specialized so as to fit them for particular kinds 

 of work; e.g., muscle has as its province the bringing about of active 

 movements. Indeed it may be said that animals are "high" 

 or " low " in proportion as the division of physiological work is 

 completely or incompletely carried out. In a high animal there 

 is great complexity of structure associated with this perfect 

 division of function, while exactly the reverse is true for a low 

 animal. And in degenerate forms, such as the Ascidians (p. 297), 

 animals derived from relatively complex ancestors have become 

 simplified so as to suit them for simpler conditions of life. The 

 loss of digestive organs by tape- worms (p. 441) is a further 

 example of the principle. 



Careful examination of any one kind of tissue shows that it 

 is entirely or largely composed of structural units known as cells, 

 comparable to the bricks and stones, &c., which make up the 

 building materials of a house, if we pursue a comparison which 

 has elsewhere been made use of. It is these cells that consist 

 of the actual living substance, protoplasm, with which vital actions 

 are associated. Each cell contains a particle of specially modified 

 protoplasm known as the nucleus, which appears to be a regu- 

 lative centre. These cells differ largely in shape according to 

 the nature of the tissue. A simple case is that of lymph (p. 41), 

 which consists of a clear fluid (plasma) in which are suspended 

 innumerable irregular lymph corpuscles, which in this instance 

 are the constituent cells. Epithelium (fig. 288) again consists of 

 one or more layers of cells closely packed together and possessing 

 various shapes in different cases. There is, for example, simple 

 scaly epithelium, made up of flattened cells united by their edges 

 and only one cell thick; this is comparable to a tessellated pave- 



