40 PHYLUM VERTEBRATA (CRANIATA). 



lated with its rigidity, and is therefore best developed in 

 those animals which have to support the body on land. 



The integument consists of two distinct layers, the epidermis 

 externally and the cutis internally. The epidermis is composed 

 of many layers of cells, of which the upper and older layers are 

 worn off, while the lower layer (stratum malpighi) is actively 

 growing, and serves as a matrix for the continual renewal of 

 the upper layers, and sometimes contains pigment. The cutis 

 is principally formed of fibrous connective tissue, with which 

 muscular elements striped and unstriped come into relation 

 without however forming a dermo-muscular envelope, as in the 

 Annelids. Some of the appendages of the skin are epidermal 

 structures (hairs and feathers). Some are derived from ossi- 

 fications of dermal papillae, which sometimes may even give 

 rise to a hard and complete dermal armour (scales of fishes and 

 reptiles, carapace of armadillos and tortoises). The epidermis 

 is derived from the ectoderm of the embryo, the cutis or dermis 

 being mainly a mesodermal product. 



The endo-skeletal tissue of the lower Vertebrata and of all 

 vertebrate embryos consists solely of cartilage (Marsipobranchii, 

 Elasmobranchii), but in most groups osseous tissue, supple- 

 menting or, in the higher forms, largely replacing the cartilage, 

 is present in the adult. 



The muscular tissue may be divided into two categories : 

 They are (1) the somatic or myotome muscles, which are derived 

 from the epithelial wall of the myotomes or dorsal segmented 

 parts of the mesoblast of the embryo, and (2) the mesenchyma- 

 tous * muscles, which are developed from the ventral part of 

 the mesoblast (wall of the splanchnocoel). The myotome 

 muscles are innervated exclusively by the ventral roots of the 

 spinal nerves, and by the third, fourth and sixth cranial nerves, 

 which are the only ventral nerve roots found in the brain. The 

 mesenchymatous (visceral) muscles, which appear to be derived 

 from mesenchymatous mesoderm, are innervated by the ventral 

 roots in the trunk, but in the head by the fifth, seventh, ninth 

 and tenth cranial nerves, which are usually regarded as dorsal 

 nerve-roots, and which contain also afferent nerve fibres (see 

 account of nerves under Pisces). The somatic muscles are cross- 



* Sometimes called visceral, but this is a bad name, as many of them 

 lie in the body-wall. 



