80 OEUANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



property, like that of movement, resides in definite tissues and organs 

 which constitute the nervous system. For those cases in which a 

 nervous system has not separated from the common contractile basis 

 (sarcode) or from the uniform cell parenchyma of the body, we may 

 suppose that the organism possesses the first beginnings of an 

 irritability serving for perception. This, however, can scarcely be 

 called sensation, for sensation pre-supposes the presence of conscious- 

 ness of the unity of the body, and this we can scarcely attribute to 

 the simplest animals without a nervous system. 



The appearance of muscles is coincident with that of the nervous 

 tissues, which are developed in connection with the sense epithe- 

 lium of the surface (Polyps, Medusae, Echinoderms). In such cases 



the nerve fibres and ganglion cells 

 which all lie mingled together keep 

 their ectodermal position and their 

 connection with the sense epithe- 

 lium. The view that the first diffe- 

 rentiation of the nervous and mus- 

 cular tissues is to be sought in the 

 so-called neuromuscular cells of the 

 fresh-water polyps and Medusae has 

 been shown by later researches to 

 be untenable. 



The arrangements of the nervous 

 system can be traced back to three 

 distinct types (1) the radial ar- 

 rangement found in the radiate 

 animals ; (2) the bilateral arrange- 

 ment found in segmented Worms, Arthropods, and Molluscs; (3) 

 the bilateral arrangement of the Yertebrata. In the first case the 

 central organs are radially repeated ; in the Echinoderms as the so- 

 called ambulacral brains or nerves, which are found in the arms and 

 are connected together by a circumoral nervous commissure contain- 

 ing ganglion cells (fig. 75). 



In the second type the nervous system, in the simplest cases, 

 consists of an unpaired or paired ganglionic mass placed in the 

 anterior part of the body above the pharynx, and known as the 

 supra-cesophageal ganglion or brain. From this centre radiate in 

 the simplest cases (Turbellaria) nerves which have a bilaterally sym- 

 metrical distribution, and of which two are larger than the others, 

 and take a lateral course (fig. 70). 



FIG. 75. Diagram of the nervous sys- 

 tem of a star-fish. N, nerve ring 

 which connects together the five am- 

 bulacral centres. 



