Yertebrata. 325 



are soft and protoplasmicj but in all other groups the outer portion 

 of the epidermis consists of cornified cells^ so that an outer 

 stratum corneum, and an inner stratum mucosum [Rete 

 Malpiyhii) , in which the cells are protoplasmic, may be dis- 

 tinguished. In the Amphibia the stratum corneum is only one 

 or two cells thick, in the higher Vertebrata thicker ; in various 

 regions of the body it is developed in different ways, and in certain 

 parts may attain a very considerable thickness and great hardness. 

 The claws, for instance, of Reptiles, Birds, and Mammals, are 

 thickened portions of the stratum corneum which surround the last 

 joint of the digit. Ecdysis in the Yertebrata consists in a 

 throwing off of the stratum corneum, either in one piece (Amphibia, 

 some Reptilia), or bit by bit. Below the epidermis is a layer of 

 connective tissue, the dermis, which varies in thickness and 

 firmness, and is connected with the subjacent parts by the loose 

 subcutaneous connective tissue; in the dermis there are 

 numerous smooth muscle cells or transversely striped muscle fibres, 

 and below, but still connected with it, there are, especially in the 

 higher Vertebrata, the continuous laminate cutaneous muscles con- 

 sisting of transverse muscle fibres. In the cells of both epidermis 

 and dermis there is frequently a deposit of pigment. Glands 

 of many kinds belong to the skin. In the Pish, between the outermost 

 cells of the epidermis, there are beaker-shaped cells which 

 secrete mucus ; true glands are not generally present, although 

 in other Vertebrata they attain a great development, sunk into, 

 or below, the dermis and opening through the epidermis. In 

 members of all the classes of Vertebrata, with the exception of the 

 Lancelet {Amphioxus), ossifications are present in the dermis 

 forming plates of varying thickness (scales of Pisces, etc.). Sometimes 

 these plates attain a considerable size, and may be united to form 

 an e X o s k,e 1 e t o n which surrounds some portion of the body 

 (some Fish, Tortoises, a few Mammals), and are often intimately 

 connected with the endoskeleton, especially in the head region. 



The endoskeleton is, at very early stages of development, 

 represented only by the notochord, a cord or rod of cellular 

 connective tissue, lying below the central nervous system. In 

 Amphioxus the adult skeleton consists almost exclusively of the 

 notochord (Fig. 293, p. 355), but in all other Vertebrates, other 

 skeletogenous structures develop ; largely around, and in connection 

 with, the chorda. They surpass the notochord in size, and it is, in 

 fact, almost obliterated by the newly-developed skeleton. The latter 

 consists partly of cartilage, partly of bone ; even when the adult 

 skeleton is principally bone, it usually consists at first of cartilage, 

 which is gradually absorbed and replaced by bone, i.e., it ossifies: 

 or it is covered by bony plates, membrane-bones, beneath 

 which it may persist : or it may disappear altogether. 



