. A manual of zoology. Zoology. GENERAL ORGANOLOGY 111 ik^^.'^if'^ ^ Locomotion among Lower Animals.—The lowest forms, the Protozoa, progress ahiiost exclusively by processes of the cell: pscitdopodia, cilia, or flagella. In the metazoa this is rarely the case. Amoeboid movements of the epithelial cells, indeed, occur in the coelenterates and in many worms, but do not suffice for change of position. More effective is the ciliated or flagellated epithelium, by which ctenophores, turljel- larians, and rotifers swim; this occurs, besides, in many larva: of animals which, in the mature state, are
. A manual of zoology. Zoology. GENERAL ORGANOLOGY 111 ik^^.'^if'^ ^ Locomotion among Lower Animals.—The lowest forms, the Protozoa, progress ahiiost exclusively by processes of the cell: pscitdopodia, cilia, or flagella. In the metazoa this is rarely the case. Amoeboid movements of the epithelial cells, indeed, occur in the coelenterates and in many worms, but do not suffice for change of position. More effective is the ciliated or flagellated epithelium, by which ctenophores, turljel- larians, and rotifers swim; this occurs, besides, in many larva: of animals which, in the mature state, are unable to change their location or do so only by the aid of muscles. Nearly all ccelenterates, echinoderms, molluscs, and the majority of the worms leave the egg-membranes as larva; swimming by means of cilia. The musculature is alone adapted for energetic motions. The arrangement of this varies with and depends upon the constitution of the skeleton. Forms without a skeleton have commonly the dermo-muscular tunic, a sac of circular and longitudinal muscle fibres which is firmly united with the skin. If a skeleton be formed by the skin, as in the arthropods, where the epidermis secretes a firm cttticular skeleton, then the sac breaks up into groups of muscles, which find points of attachment upon it; if, on the other hand, as in the vertebrates, an axial skeleton be formed, fixed points are fur- nished for muscular attachment, so that the musculature olstains a new character, in particular a deeper position. A unique locomotor apparatus is the amhulacral system of the echinoderms, a system of delicate litde tubes with protrusible por- tions which function as feet, described in connection with that group. Electric Organs.—In several fishes the muscles at certain places are modified into electric organs, which, in Malaptentnis, Tor- pedo, Gymnotus and Astroscopus can give ener- getic discharges; in Raia and Mormyrus the discharges are weak and cannot be felt by man. Each organ
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
