CHORDATA, which possess the following characters. The integument is composed of an epidermis consisting of several layers of cells, and a dermis chiefly composed of fibrous connective tissue. There is a well-developed endo-skeleton, consisting of a cranium with moveable jaws: a backbone with neural and sometimes haemal, arches, and vertebral centra replacing the notochord more or less completely: and two pairs of limbs, pectoral and pelvic. The nerve-cord is enlarged anteriorly, forming the brain which is lodged within a cranium. There are two olfactory pits and two eyes. The lens of the eye is cellular, and developed from epiblast: the retina is cup-shaped, and derived from a vesicle of the brain. A contractile fibro-muscular iris, placed in front of the lens, controls the amount of light entering the eye. There are also two auditory organs arising as involutions of the epiblast which close and usually become highly complicated. A ventrally placed section near the head of the closed blood-vascular system is differentiated into a rhythmically contractile heart. There is a set of haemoglobin-containing blood-corpuscles or haematids, with fixed outlines in addition to the amoeboid corpuscles, or leucocytes.
There is a blood-making organ - the spleen - lodged in the meso-gastrium and traversed by the blood-current. And there is a system of lymphatic channels communicating with the blood-vascular system. The coelome is large and lodges the heart, the main portion of the alimentary 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 though 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.
1 In the Cyclostome Petromyzon the post-anal gut is represented by a solid rod of cells. The blastopore persists as the anus; it is, at first, dorsal, but shifts to the ventral surface in consequence 'of the elongation of the embryo and the formation of the tail.' Shipley, P. R. S. xxxix. 1885.
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 - subcutaneous tissue - in which fat-cells, scattered or in masses, may be developed 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 Vertebrata. 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 musclefibres take various directions in accordance with the necessities of their position. The muscles of the limbs are derived from the primitive muscle-plates (p. 334) in Elasmobranchii, and probably in other orders of Fish as also in Amphibia and Lacertilia. In higher Vertebrata they arise independently, probably by an abbreviation of development. All the muscles referred to above lie externally to the ribs and the transverse processes of the vertebrae. They are hence termed episkeletal.
A superficial portion of this episkeletal system may retain its original independence of the skeleton and forms the 'muscles of the skin.' It is sparingly represented in Pisces, in Amphibia and Reptilia, except where moveable scales are present, as in Gymnophiona and Ophidia. In Aves it is said to form the erectors of the feathers, and in most Mammalia it is very well developed. The Primates, however, retain but few traces of it (platysma myoides). In Amniota there is a small system of hyposkeletal, or sub-vertebral muscles, underlying the vertebral column, the origin of which is unknown. It is also uncertain whether or not the fibro-muscular diaphragm of Mammalia belongs to this last-named system. Both epi- and hypo-skeletal muscles are striated. On the other hand, the musculature of the viscera with the exception of the heart, is, as a rule, non-striated.