INTEGUMENT. 690 INTEGUMENT. cellular epitlieliuiii, and U'tu'iitli it a layer of connective tissue of niesencliyniatous origin. In the latter the skejetiil stniclurcs develop. In some forms (Crinoidea, Ophiuroidea) , purtii'u- hirly in adult stages, there is no sharp line of dis- tinction hctwcen epithelium and cutis. The epi- thelium is covered by a cuticle, and in Asteroidea and Kchinoidea it is ciliated over the whole sur- face; in other forms usually the foiiilf.'roove alone is ciliated, rnieelliilar <.'lands and pi^Mneiit cells olso (jccur in the epitlii-liuni ; while sensory cells, nerve-gunjilia, and lihres dilTerentiate out of it. The integument of annelids consists of the hypo- derniis or i)o<iy epithelium and an outer cuticle. The latter may he thin, with pores for cilia to pass through, or it may Ix' a thick protective cov- ering and oiler supiiort for the attachment of muscles. It may he cliitiiious or even calcilied into a hard shell, and is either jjroduced liy gland cells in the hypodermis or else it is a proiluit of the metabolism of these cells. The hypodermis may be .so much reduced as to be little more than a strong cuticle, as in the case of the Polyzoa, where it is hardly recognizable. A basal mem- brane sometimes underlies the hypodermis. AKTiiiioroDS. The chitinous secretion of the hypodermis is still more strongly developed in arthropods than in worms. It covers the whole surface of ilie bo<ly and ap|K'ndages, and is known as the 'e.xoskeleton ;' and to it the muscles are attached. As ita jiresenee is a barrier to the increase in size of the animal, it is thrown off from time to time, and its place is taken by a .soft, flexible coating which has developed under- neath it. The new coiiting soon hardens when exposed on the surface. See Moi.tino. Vertkbhates. The integument or skin of verte- brates IS an organ of nmch extension, but of little thickness. Physiologically it is <me organ he- cau.se its parts are closely imited togi'ther and act together, and in most vertebrates they are easily separable from underlying tissues. The intx'gvmient functions as an organ of sup|Kirt and protection to internal tissues, and as an organ of secretion and excretion. Morphologically it is made up of organs of widely dissimilar origin and histological character. It is divisible, how- ever, into two layers. The out<rmo><t of the two is derived wholly from the ectoderm, and consists of an epithelium of cuboidal or flattened cells. The inner layer is derived from mesen- chyme, and consi.its chiefly of fibrous connective tissue in which there is a large amount of se- creted siibst.ince. The outermost layer is called the epidermis, the innermost the derma (or der- mis) ; sometimes also 'corium' or 'cutis.' Each of these parts contains other organs. Three ttiv"" of epidermis may be distinguished: (1) That of Amphioxus; (2) that of fishes; and (3) that of the .Vnmiota. The Amphibia occupy a middle ground t>etween the second and third, in that their epidermis is at first fish like and later similar to that of the Anmiota. In the first type the epidermis consists of a single layer of cuboidal cells — the condition of the epidermis in all invertebrates. This tyjio may be called 'uni- stratous.' The second and third types differ from the first and agree with each other in that they possess more than one layer of cells; hence they may he called 'multistratous.' The epidermis of fishes consists of many layers of oohininar or cuboid cells, the more columnar ones lying at the base. There is a cuticule over the free surface provided with fine pore canals. From the essential similarity of the epidermal cell* throughout, the epidermis cannot be divided. This type may therefore be called 'hoiiioslrat<ius.' The epidermis of animals that live in the oir (. iniota) has become modified to withstand desiccation. There may now be distinguished in i.ian, for instance, two regions in the epidermis, a deep one comjK)sed of active cuboidal cells, and a more superficial one of llatteiied lornilied cells. The first is known as the 'stratum jiiuco..>um,' the second as the 'stratuin coriieuni.' These cornlllcd ctlls have become such thrcaigh gradual llatteiiing of the deefM'r-lyiiig cells, accompanied by a change ill chemical constitution by which the cells are metamorphosed into keratin. These cells are gradually worn off, and are replaced from the layers below. In seeking a knowledge of the origin of the nianyla'jered epidermis we must study the com- parative ontogenetic and phylogenetic dcvelo))- nient of cpideniiis. At the time of gastrulation the ectoderm of mammals, from which alone the epidermis 's derived, is unistratous. In this condition it is like the epidermis of all inverte- brates. In this condition, too, it persists in Amphioxus to an adult st;ige. Quite early in the emljryonic development of mammals, how- ever, the nuclei arrange themselves in two layers as though forced to do so by pressure. Such a condition prevails in the human embryo of two months. (See EMnRYOi.OQY, Hitman.) The more superficial layer consists of fiattened, transpar- ent, hexagonal elements — the deeper hiyer of smaller cells: so that already a separation into a 'stratum corneum' and 'stratum mucosum' has ."ippeared. This partly differentiated conililioii of the epidermis occurs in the amphibian larv;r. In the adult Amphil)ia the epidermis m:iy become many-layered. The outer layer is c:ist fnni time to time in one piece in Amiihibia and Reptilia. In birds and n;ammais the cornified c<'lls are be- ing constantly shed. In Amphioxus and fislies the epidermis is provided with a euticula, while in some larval forms ami in certain regions of adults it is ciliated. The epidermis is separated fiom the cutis by a basement membrane. Types or Derma. The derma is composed of connective ti.ssue and is derived from mesen- chyme. Tliis is a more compact layer than the looser subdermal tissue that lies still deeper. Tliree tyj>es of derma may he distinguished : ( I ) That of Amphioxus, consisting of a layer of fiat cells, together with a gelatinous substance which they havt secreted from their outer surfaces; I'i) that of fishes and amphibians, in which the bundles of connective tissue run in two direc- tions, parallel and ]>erpondieular to the surf;ic<>; (3) that of birds and mammals, in which the above described distribution of connective tissue bundles is lost, and the boundary between derma and epidermis is no longer straight, but strongly corrugated. The derma sends up projections or papillae into the epidermis. The derma of man belongs to this type, but in its ontogenetic de- velopment it passes through stages much resem- bling the first two. Within the integument various structures are differentiated, such as pigment in granules or specialized cells; glands, both unicellular and multicellular — scutes or scales of reptiles, feath- ers of birds, hair, nails, hoofs, and claws; and dermal bones and teeth.
