GENERAL ORGANOLOGY. 121 



to sperm which has been formed in the testes of the same indi- 

 vidual. 



Animal Organs. 



I. Organs of Locomotion. 



Voluntary Locomotion. The power to change their location 

 voluntarily is a peculiarity so prominent in animals that to the 

 general public it is sufficient for deciding whether an organism 

 belongs to the vegetable or to the animal kingdom. On this 

 account it is necessary to call attention to the fact that numerous 

 animals lose the power of locomotion, becoming fixed to the 

 ground, to plants, or to other animals. All sponges and corals, 

 most hydroid polyps, and the crinoids among the echinoderms, 

 have actively swimming larvae, but become fixed in the adult and 

 thus obtain such a marked similarity to plants that, although true 

 animals, they were long regarded as plants. Further, many mol- 

 luscs and worms are firmly fixed by their shells; indeed, many 

 crustacean forms, the cirripeds, have completely lost their free 

 motility. But a more careful investigation in all these cases will 

 show that the power of moving the separate parts exists, for the 

 corals can retract their tentacles, the cirripeds their featherlike 

 feet, and the clam can close its shell. 



Locomotion among Lower Animals. The lowest forms, the 

 Protozoa, progress almost exclusively by processes of the cell: 

 pseuclopodia, cilia, or flagella. In the metazoa this is rarely the 

 case. Amoeboid movements of the epithelial cells, indeed, occur 

 in the coelenterates and also in many worms, but do not suffice 

 for change of position. More effective is the ciliated or flagellated 

 epithelium, by which ctenophores, turbellarians, and rotifers 

 swim; this occurs, besides, in many larvae of animals which, in 

 the mature state, are unable to change their location or do so only 

 by the aid of muscles. Nearly all coelenterates, echinoderms, 

 molluscs, and the majority of the worms leave the egg-membranes 

 in the form of the planula, i.e., as a larva swimming by means of 

 cilia. 



Locomotion among Higher Animals. 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 i dermo-muscular tunic/ a 

 sac of circular and longitudinal muscle fibres which is firmly united 



