336 THE ANIMAL KINGDOM. 



canal, and genital glands, which are retained in situ by folds of its lining' 

 membrane, the peritoneum. The two kidneys lie on its dorsal aspect, behind 

 the peritoneum. They are compact glands thoiigh developed segmentally, and 

 open externally, each by a common longitudinal canal. The ova are very 

 generally shed into the coelome, taken up and carried away by two oviducts, 

 and the testes connected to a portion of the kidney through which their 

 products are conveyed to the exterior. The anal, urinary and reproductive 

 apertures are ventral and close together, opening primitively into a common 

 cloaca. 



The deeper layers of cells of the epidermis are protoplasmic and 

 form the stratum Malpighii, or rete mucosum : the more superficial layers 

 are modified, and are often converted into flat horny coherent plates the 

 stratum corneum which is least developed in aquatic Vertebrata. Certain 

 of the epidermic cells may become glandular, or pigmented, and others 

 modified into sensory nerve-endings (infra) ; and invaginations of epidermic 

 cells into the cutis in higher Vertebrata form glands of various kinds. The 

 dermis, cutis, or corium, consists of fibrous connective tissue, with connective 

 tissue cells often modified into pigment cells. It gives support both to 

 blood-vessels and nerves, as well as attachment to muscles. The spaces in 

 the cutis give origin to the superficial lymphatics. The cutis is connected 

 to underlying parts as a rule by more loosely arranged fibrous tissue sub- 

 cutaneous tissue in which fat-cells, scattered or in masses, may be de- 

 veloped in higher Vertebrata. The integument gives origin in many 

 instances to an exo-skeleton, which may be derived either from the epidermis 

 or the dermis exclusively, or in part from one, in part from the other. An 

 epidermic exo-skeleton is represented by the stratum corneum, and by 

 special developments such as hairs, feathers, scales, tortoise-shell, hoofs, 

 nails, claws, and other structures noticed where they occur. The dermic 

 exo-skeleton consists of bony scutes or plates found in a few Mammals 

 and many Reptiles, and in the scales of some Fish (most Teleostei, Dipnoi}. 

 A mixed exo-skeleton is seen in the various forms of teeth, with enamel 

 derived from the epidermis, and dentine with cement derived from the 

 dermis, and in the tooth-like scales of Elasmobranchii, Ganoidei, and some 

 Teleostei. 



The muscular system is well developed. The primitive division into 

 somites or myomeres, with intervening fibrous septa or myocommata', is 

 retained in the body-walls of Pisces, in the dorso-lateral parts of the body 

 of perennibranchiate Amphibia and Lacertilia, and in the tail of all Verte- 

 brata. It is retained in the trunk of the higher forms only in the spinal 

 muscles (and the intercostals ?) and more or less in the recti abdominis. In 

 these cases the primitive longitudinal direction of the muscle-fibres persists. 

 In the region of the head, and in the trunk generally, both in Amphibia 

 and higher Vertebrata, the primitive arrangement is lost, and the muscle- 



