448 GENERAL BIOLOGY 



Nerve and muscle. Pseudopodia, cilia and other organs 

 of outreach in the protozoa fulfill both sensory and motor 

 functions. Separate classes of organs for these functions 

 are not differentiated; and probably they were not differ- 

 entiated at first in the metazoa. Nerves and muscles must 

 have originated near the surface of the body, for only there 

 could they have been in communication with the outside 

 world. It is there we find them first in the phylogenetic 

 series, underlying the ectoderm in the hydra (fig. loi, page 

 159). It is from the underlying layers of the ectoderm, as 

 we have seen, that the nervous tissues arise in the ontogeny 

 of the vertebrates. 



It is the primary function of the nerve cell to develop out 

 of its own cytoplasm such parts of out-reach as shall serve 

 for intercommunication between all of the other tissues and 

 with each other. They must be connected externally with 

 the sense organs of the body and internally with the muscles 

 to the end that the sensing of the things of the external 

 world may bring about prompt and appropriate measures 

 for dealing with them. Muscle cells must therefore be 

 developed in connection with nerves. They are elongate 

 cells of special contractility, connected with whatever solid 

 supports the body may possess. They become aggregated 

 together and integrated into muscles of increasing strength 

 and size as the skeleton develops, giving them points of 

 insertion, and offering strength to resist their pull. 



Ganglia. — The possession of long organs of cell out-reach 

 (fibers) made it possible that nerve cells should be removed 

 from the surface where they develop, into the interior of 

 the body, where better protected from injury. The impor- 

 tance of the regulatory function they fulfill made this highly 

 desirable, and intercommunication between the cells 

 themselves was facilitated by the assembling of them togeth- 

 er in groups. The simplest of these we know as ganglia. 



