42 COMPAEATIVE ANATOMY. 



them that the nerves of the higher sensory organs arise, which in 

 position and direction have one widely-distributed arrangement. 



From this form another is directly derived, determined apparently 

 by the well-marked metamerism of the body. Whilst in unseg- 

 mented animals possessing an oesophageal ring, the ventral parts of 

 the body are supplied by nerves which arise from the suboesophageal 

 ganglia, we find that the number of ventral ganglia is increased when 

 the body is broken up into parts lying one behind the other (meta- 

 meres). A ventrally-placed series of ganglia is formed by the 

 development of a separate pair of ganglia for each segment ; and 

 these, being united to each other by longitudinal commissures, form 

 a ganglionic chain. The Ringed worms and the Arthropoda present 

 us with this form. Further differentiation gives rise to all kinds of 

 variations of this type. 



In the first place, the size of the ganglia varies with the size of 

 those parts of the body that have to be innervated ; and in the 

 second place, the ganglia of several segments of the ventral cord 

 fuse into larger ganglionic masses. 



Even when the central nervous system is entirely dorsal, as in the 

 Vertebrata, it undergoes differentiations of this kind. When the 

 most anterior part of the body is developed into a head, the most 

 anterior part of the central nervous system is developed into a special 

 region, the brain, which is marked off from the remainder of the 

 medullary tube, or spinal cord, which remains more equal in 

 size throughout. As differentiation advances, variously developed 

 regions appear in the brain. 



e) Sensory Organs. 

 § 36. 



The sensory organs inform the organism of the condition of 

 the outer world. Protoplasm, in its indifferent condition, charac- 

 teristic of the lowest organisms, reacts to various stimuli from with- 

 out, and appears to be the seat of the lowest kind of sensation. 

 When the surface of the body is not completely marked off from the 

 inner portion of the organism (Rhizopoda) it is used as an organ of 

 perception, of course of the very lowest grade ; it functions therefore 

 as a sensory organ of the lowest order. When the surface is more 

 distinctly marked off, and a distinct outer layer of the body is 

 established (Infusoria, Gregarinas), we get a differentiation of great 

 importance for sensory perception. 



Although, indeed, particular parts of the surface in the Infusoria 

 specially acquire the function of sensory organs, yet there is no 

 ground for speaking of sensory " organs " in an anatomical sense, in 

 this case, any more than there is in the still lower stages. Sensory 

 organs only appear when a nervous system is marked off, for sen- 

 sory organs are the end-organs of the sensitive nerves. 



