148 Elementary Zoology. 



diffused nervous system has preceded a differentiated central 

 and peripheral nervous system. In Hydra there are nerve- 

 cells scattered over the ectoderm, in association, chiefly at any 

 rate, with the cnidoblasts. A diffused nervous system (asso- 

 ciated, however, with a concentrated central nervous system) 

 has survived in certain simply organized worms (the Nemer- 

 tines) where there is a layer of nervous tissue completely 

 surrounding the body. Even in the embryo frog there are 

 more than traces of the same. The epiblast of the young 

 embryo is divided into an outer layer, from which the future 

 epidermis is to be derived, and a deeper layer from which the 

 central nervous system is developed. This layer, however, is 

 not found only where the central nervous system will be 

 ultimately produced, but it is continuous right round the body, 

 and is generally held to represent an archaic state of affairs, 

 where a continuous nerve-sheath was the nervous system. 

 Even in the same group to which Hydra belongs the com- 

 mencement of a central nervous system is to be seen. Round 

 the margin of the "umbrella" of Medusse are special concen- 

 trations of nerve-cells, the forerunner of a central nervous 

 system. In all the higher animals described in the present 

 book a central nervous system is present ; and in all of them 

 it is removed from the epidermis, though — equally in all — it 

 is developed from the epiblast of the embryo. 



The central nervous system exists in these animals, it will 

 be observed, in two rather different forms. In all the inverte- 

 brate types it consists of a supra-oesophageal ganglion or 

 ganglia, connected with a suboesophageal pair of ganglia, or 

 chain of ganglia by connectives that pass round the gullet. In 

 the vertebrates, on the other hand, the central nervous system, 

 consisting of the brain and spinal cord, is entirely dorsal in 

 position, the alimentary tract being wholly ventral to it. There 

 are, furthermore, these additional differences between the two 

 types. In the vertebrates the central nervous system is not 

 cut up into masses of nerve-cells connected by tracts of nerve- 

 fibres, into ganglia ahd connectives ; secondly, it encloses a 

 central canal. The nervous stem of vertebrates is a tube of 

 nervous matter. In the invertebrate the ventral and chief part 



