GENERAL ORGANOLOGY 



111 



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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 is formed of 

 columns of numerous superimposed plates separated by connective tissue. 

 Each plate is a metamorphosed muscle fibre, the side to which the nerve is 

 attached forming the negative pole. 



II. Nervous System. 



Scarcely a system of organs shows such a regular progression as the 

 nervous system. The different stages may be termed the diffuse, the 

 linear, the ganglionic, and the tubular types. 





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-AW 



BG 



Fig. 76. — Diagrammatic sec- 

 tion of electrical apparatus (from 

 Wiedersiieim). Ttie arrow points 

 dorsally or anteriorly. BG, con- 

 nective-tissue framework; EF , 

 electrical plates; G, gelatinous 

 tissue ; N , nen.-es entering through 

 the septa; A^A^, nerve termina- 

 tions. 



