14 General Part. 



described. The more general characteristics of these organs will now 

 be studied. 



There is a very slight differentiation of organs in the lowest Metazoa, the 

 Coelentera ; but this group is peculiar in many respects (see Special Part). 



1. The Skin. 



The skin which forms the external limit of the body consists, in 

 the simplest cases, of an epithelium only, the epidermis. It 

 frequently includes, however, a layer of connective tissue also, the 

 dermis. In most Metazoa the epidermis is a simple epithelium, 

 consisting of squamous or columnar cells, and often ciliated; in the 

 Vertebrata alone, it is a stratified epithelium, of which the outer 

 cells are horny and thus form a protective covering for those beneath : 

 this is represented in many other Metazoa by the cuticle (Chsetopoda, 

 Insecta, and others) . The dermis is a layer of connective tissue of 

 varying thickness, and of a firm consistency, lying beneath the 

 epidermis ; it is not generally sharply marked off from the neighbour- 

 ing structures, in most Vertebrates it passes gradually into the loose 

 sub-cutaneous connective tissue. In the lower Metazoa calcareous 

 deposits of different sizes and shapes are frequent in the dermis 

 (Echinoderms) and among the Yertebrata there are often bony plates 

 (scales of Fish). Muscle cells also are very common. 



Glands, both unicellular and true glands, of diverse function, are 

 often found in the skin, such as mucous glands, stink glands, oil or 

 sweat glands. Various appendages, such as hairs or bristles, may 

 be present, but under these names are comprised structures of very 

 different kinds. The c h 8e t ffi of Ohaetopoda, for example, are solid 

 cuticular processes, arising as a secretion from certain epidermal cells ; 

 the s e t SB of Arthropods are also cuticular, but they are hollow, and 

 contain an epidermal core. Mammalian hairs, on the other hand, 

 consist of horny epidermal cells. 



Suckers are specially modified muscular portions of the skin, serving in 

 many animals as organs of adhesion. They usually stand out from the sua-face 



in the form of small thick-waUed 

 A B cups with smooth edges, the convex 



'i 1^, — ^ I ■ . - ■ • side towards the skia, and the con- 



cave side free. The sucker generally 

 •' ^'^~ '■^,/,\ works somewhat in the following- 

 f ■ '<>:\ way: its edge is pressed agamst a 



foreign object (Pig. 16, A), and by 

 the contraction of the muscles 



r^- ic T^- ™ I 1 • i. abundant in its walls, the space 



Fig. 16. Diagram of a sucker in two i. j. • > ^ ojja,v-o 



different conditions ; transverse sections. h between the two mcreases (Fig. 16, 

 skin, s sucker, «. foreign body. See text. — B), so that a chamber is formed 

 Orig- wherein the pressm-e of the air or 



water is sub-normal, while the ex- 

 ternal pressure on the outer wall of the sucker holds it firmly to the object. 



Suckers of this description occur in Plat-woi-ms, Leeches, Cuttle-fish, some 

 Mammals, etc. . but other adhesive organs are also found. Some animals 





