THE NERVOUS SYSTEM OF CCELENTERA. 



SINCE Oscar and Richard Hertwig published their 

 extremely valuable monograph on the sea-anemones, 

 when the nervous system both in the ectoderm and the 

 endoderm of these animals was for the first time accurately 

 described, a series of important papers has been published 

 both at home and abroad confirming their general results 

 and extending them to other groups of Coelentera. The 

 anatomical details of the nervous system of a group of 

 animals which must still be regarded as the most primitive 

 of all Metazoa naturally excites the interest of all zoologists 

 and physiologists. The complicated nerve ganglia and 

 cords of Chsetopoda, Arthropoda, Mollusca and Vertebrata 

 are in nearly all cases situated in the mesoblastic tissues of 

 the adult. The teaching of embryology points undoubtedly 

 to the ectoderm or epiblast as the source from which this 

 nervous system is derived. Therefore must we look to 

 those animals which do not possess any mesoblast at all if 

 we wish to learn more about the original form the nervous 

 system of animals possessed. 



In the higher Metazoa the greater part of the nervous 

 system is not difficult to discover. 



The greater ganglionic enlargements, and the bundles 

 of nerve fibres forming nerve cords from which the finer 

 branches pass to the muscles and the organs of sensation, 

 stand out clearly from the surrounding tissues. But in 

 some forms, such as Nemertine worms, Platyhelminthes 

 and in the embryonic stages of many others, including 

 several Vertebrata, the main trunks of the central nervous 

 system not only give off definite nerve fibres to definite 

 organs or tissues but are connected with a loose, spongy 

 plexus of fibrils and cells which spreads through the tissues 

 in a manner that it is difficult to fully map out. 



In most cases such as Palaeonemertines, Nematoda, 

 Tadpoles, etc., this plexus is situated immediately below 

 but continuous with the ectoderm, and it is reasonable to 



