LOWER ORDERS. The cell integument found in certain lnfusoria consists of a chitinous or cellu lose membrane, which may PVC!' harden into a shell by the impregnation of carbonate of lime. lu Dililugia this membrane becomes a case by its onion with small foreign particles. The integu n.ent or body epithelium (epidermis) of the Cnidaria consists of one layer of epithelial cells. These cells may he eiliated or flagellate, and in this layer occur the stinging cells or nematocysts (q.v.). These stinging cells may be grimi)ed in masses, in which ease they are known as 'stinging knobs' or stinging, batteries. Moreover, in the ectoderm sensory, nerve, muscle, gland, and pig ment cells may arise. In the Scolecida, a body epithelium occurs among fiat-worm; only in Tur bellaria. It is ciliated and in it are glands, the so-called 'rods' or rhabdites. In trematodes and cestodes its place is taken by an elastic cuticular membrane, usually perforated by fine pores. In nematodes and Sagitta it is a single layer. in some species ciliated. The entire outer surface of mollusks is covered by a single layer of epi thelium whieh may be ciliated in regions not pro tected by a shell. In this layer are many unicel lular glands or the ducts of others that have sunk below the surface of the epithelium. The shell of mollusks is a calcified cuticle. Stinging cells may occur in the integument of gastropods, al though the pigment usually occurs in the deeper cutis. Th, integument of cephalopods is a cylindrical epithelium, and is made up of deep lying connective tissue which contains contracting and expanding pigment cells. the chromatophores. The integument of echinoderms consists of a uni cellular epithelium, and beneath it a layer of connective tissue of in•sencliyinatous origin. In the latter the skeletal structures de‘clop. In some forms (l'rinoidea, I Ipliitiroidea), particu larly in adult stages, there is no sharp line of dis• tinction between epithelium and (gni,. The epi thelium is covered by a cuticle, and in Asteroidca. and Echinoidea it is ciliated over the %%hole sur face; in other forms usually the food-gr(sive :thaw is ciliated. l'iticel lular glands and pigment cells also occur in the while sensory cells, nerve-ganglia, and fibres differentiate out of it.
The of annelid; consists of the hypo (lentils or iaaiy epithelium and :In outer cuticle. The latter may Ile thin, with pores for cilia to pass through, or it may be a thick protective eUV• ming and offer support for the attachment of muscles. It may be chitinous or even calcined into a hard shell, and is either produced by gland cells ln the hypoderIllim 1)1' eke it is a product of the metabolism of these cells. 'flte hypodermis may be so nitwit reduced as to be little more limn a strong cuticle, as in the case of the Polyzon, where it is hardly recognizable. A basal mem brane sometimes underlies the hypoderm's.
AftTIIIMPOOS. The (11111111MS ion of the hypoderm's is still more strongly developed in arthropods than in worms. It covers the whole surface of the body and a phslidages, and is known as the `exoskeleton:' and to it the muscles are attached. As its presence is n barrier to 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 coating soon hardens when exposed on the stir face. See Alot.T1XO.
Viairratit.vrEs. The integument or skin of verte brates is an organ I if much extension, but of little thickness. Physiobvienlly it is one organ be cause its parts are closely united together and act together, and in most vertebrates they are easily separable front mulct-lying tissues. The integument functions as an organ of support and protection to internal tissues, and as an of secretion .and excretion. it is made up of organs of widely dissimilar origin and histological character. It is divisible, how ever, into two layers. The outermost of the two is derived wholly front the ectoderm• and consists of an epithelium of euladdal or flattened tills. The inner layer is derived front mesen chyme, and conskts chiefly of fibrous connective tissue in there is a large amount of se creted substance. The outermost layer is called the epidermis, the innermost the derma (or (ler ; sometimes also 'corium' or 'clitis.' Each of these parts contains other organs.