THE DISPLAY OF ENERGY 



375 



..oral cUia 



...orccl ^roov(2^ 



...Contractile 



vcxcixol<2^ 



in animals. It will not be amiss, however, even at the risk of repeti- 

 tion, to take up, from the standpoint of function, the effects of some 

 of the forms of animal behavior. 



In simple animal cells, such as Ameba, the outer portion of the 

 cell is in contact with the stimulus which is transmitted through the 

 protoplasm to the in- 

 terior of the cell. In 

 cells with cilia, continua- 

 tions of these structures 

 that reach down into the 

 protoplasm apparently 

 act as organs for recep- 

 tion of stimuli. Euglena 

 has a pigmented "eye- 

 spot" which is definitely 

 sensitive to light. In 

 some specialized proto- 

 zoan cells a motoriuni or 

 co-ordinating center is 

 found. 



In higher forms of 

 animals there are defi- 

 nite receptors in the form 



of sense cells, organs which act as stimulating centers with nerves 

 serving as conductors to the parts that are fitted for resi)onse, the 

 so-called effectors. Examples of such effectors are the muscle cells, 

 gland cells, and the cells of such organs as the luminous areas of the 

 fire-fly, and the electric organ of the electric eel. In the nerve net of 

 such animals as Hydra, or the jellyfish, apparently no synapses 

 exist between the cells, the nervous system being a tangled net through 

 which the nerve impulse flows. In such a nervous apparatus the 

 nerve activity is slower than in a type of nervous system found in 

 animals like the earthworm. The so-called "ladder nervous system" 

 exists in worms and in arthropods generally, and is seen at its highest 

 development in the insects, where there is a series of units in which 

 the neurons are connected by synapses. Such types of nervous 

 systems are more effective because the nerve impulses travel only in 

 one direction through a neuron, while in the nerve net they may travel 

 in any direction. Receiving neurons in the sense organs are found 

 at the surface or in a situation where they may be exposed to stimuli. 



H. w. H. — 25 



.caudal Cirri 



Euplotes, a hypotrichous ciliate. Note the 

 thickened cilia or cirri by means of which the 

 animal is able to make siuUlen jumping move- 

 ments. 



