COMPARATIVE. HISTORICAL. 541 



In molluscs, the skin, consisting of epidermis and corium, is intimately united 

 with the underlying muscles to form the musculo-cutaneous tube of the body. 

 Cephalopods have, in their skin, the so-called chromatophores; that is, round 

 cells filled with granular pigment, at the periphery of which muscular fibers are 

 attached in a radiate manner, so that their contraction must increase the colored 

 surface. Through the play of these muscles there result the color-variations 

 observed in cuttle-fish. Chromatophores are present, also, in other classes of 

 animals, such as amphibia (frog) and fish (pike). In these animals, they appear 

 as connective-tissue cells, within which pigment-granules either collect toward 

 the center or swarm toward the periphery, while the processes of the cells them- 

 selves do not change their place. Every cell is provided with numerous nerve- 

 endings, which surround the pigment-mass in the form of garlands, with free 

 terminal radiations. Special glands furnish the material for the formation of the 

 scales of the snails. In all invertebrates, the development of the scales takes 

 place from a portion of the surface of the body of the animal that has been desig- 

 nated the mantle. 



In articulates, the entire surface of the body is covered by a more or less 

 solid shield, which is to be considered as a cuticular structure consisting of chitin, 

 which is separated from the underlying matrix. It extends for some distance 

 into the digestive tube and the trachea. In the formation of the skin it is thrown 

 off and replaces itself anew 7 from the matrix. This shield, which affords protection 

 to the body, serves, at the same time, for the attachment of the muscles. It 

 thus becomes a passive motor organ comparable to the skeleton of the vertebrates. 



The echinoderms exhibit deposits of lime in their skin, in consequence of 

 which they often acquire a cutaneous skeleton. The deposits of lime are either 

 united to form large immovable plates, as in the scale of the sea-urchin, or united 

 together in segments, as in the arms of the star-fish. In holothurians alone, the 

 significance of calcification with respect to the cutaneous skeleton is of subordinate 

 importance. In them, only isolated plates of lime have remained in various 

 forms. In worms, the skin forms with the underlying muscles the musculo- 

 cutaneous tube. The epidermis is, in some, provided with cilia; in others (tape- 

 worms) it is traversed by pores; while in still others it is without any appendage. 

 The hooklets on the head of teniae, the rod-shaped motor bristles on the body 

 of earth-worms, are cuticular formations. Cutaneous glands are present in the 

 more highly developed worms, such as the leech. 



The integument of ccelentrates (zoophytes) is characterized by the forarunners 

 of disseminated nettle-cells; that is, cells provided with whip-like processes, which 

 contain a corrosive fluid and serve as organs of capture. Cilia are present in many; 

 in some a tubular, external, chitin-like skeleton is formed. The integument of 

 sponges is suggestive of that of zoophytes. Infusoria possess numerous cilia, which 

 in part are even subject to voluntary stimulation. Rhizopods are wholly unpro- 

 vided with a true skin. Nevertheless under these circumstances the formation of 

 silicious (radiolaria) or calcareous structures (monothalamia and polythalamia) 

 is noteworthy. 



Historical: Hippocrates (born 460 B. C.) and Theophrastus (born 371 B. C.) 

 distinguished perspiration from sw-eat. According to the latter the secretion of 

 sweat stands in a certain antagonistic relation to the secretion of urine and to 

 the amount of water in the feces. Individuals suffering from fright were believed 

 to sweat more freely from the feet. Father Augustinus stated that he knew an 

 individual who was able to sweat voluntarily. According to Cassius Felix (97 

 A. D.) the skin absorbs water in the bath.- He made investigations into the 

 evaporation from the skin. Sanctorius (1614) measured more accurately the 

 insensible perspiration and the loss of weight on the part of a fasting individual. 

 The hair-follicle and the root of the hair are mentioned in the Talmud. Alberti 

 (1581) recognized the hair-bulb. Donatus (1588) made the first report of sudden 

 graying of the hair. Riolan (1626) discovered the cutaneous pigment of the 

 negro in the epidermis. De Heyde (1684) and Leeuwenhoeck described the 

 ciliated movement on the beard of mussels (1694). 



