Februakt 15, 1918] 



SCIENCE 



157 



proved to exist in a number of animals. 

 They were definitely identified by Bethe 



(1903) in jelly fishes, by Wolff (1904) 

 and by Hadzi (1909) in hydrozoans, and 

 by Groselj (1909) in sea-anemones. In 

 fact the coelenterate nervous system seemed 

 to be nothing but a nerve-net. Von 

 Uexkiill's phj-siological studies led to much 

 the same conclusion concerning the ner- 

 vous system of echinoderms. Prentiss 



(1904) in a bi'ief summary gathered to- 

 gether the evidence to show that nerve- 

 nets were at least components of the 

 nervous systems of worms, arthropods, 

 mollusks and even vertebrates (Fig. 3) 



Fig. 3. Outline of a nerve-net from a vertebrate 

 blood-vessel (after Prentiss). 



where they were especially associated with 

 the blood vessels (Dogiel, 1893, 1898; 

 Bethe, 1895, 1908; Leontowitsch, 1901; 

 Cavalie, 1902; Prentiss, 1904), including 

 the heart (Dogiel, 1898; Hofmann, 1902; 

 Bethe, 1903). Moreover, it is now re- 

 garded as probable that there is a nerve- 

 net associated with the musculature of the 

 vertebrate digestive tube. Thus nerve-nets 

 were identified from the ccelenterates to 

 the vertebrates and some of the more 

 ardent advocates for this type of nervous 

 organization went so far as to assume that 

 it was the only type of nervous structure 

 really extant and that the evidence for a 

 synaptic sj'stem rested upon artifacts that 

 obscured the real relations of cell to cell. 

 But this extreme position has not been 

 justified by further research. It is now 

 generally admitted that the conceptions of 

 a synaptic system and of a nerve-net are 



not opposing ideas, but represent two types 

 of nervous organization, both of which may 

 exist side by side in the same animal (Her- 

 rick, 1915; Bayliss, 1915). Judging from 

 the fact that the nerve-net is apparently 

 the exclusive type of nervous organization 

 in the ccelenterates and that it becomes 

 progressively less and less evident the 

 higher one ascends the animal series, it 

 seems fair to conclude that the nerve-net is 

 the more primitive type and that in the 

 course of evolution it has given way more 

 and more to the synaptic sj'stem which has 

 finally come to be the dominating plan of 

 nervous organization in the higher animals. 

 From this standpoint one of the higher 

 metazoans might perfectly well possess both 

 types of nervous structure : nerve-nets hav- 

 ing been retained in its more conservative 

 portions and synaptic structures having 

 been developed in its more progressive 

 parts. Thus the nerve-net may be re- 

 garded as phylogenetically older than the 

 synaptic system. 



If this view of the relation of nerve- 

 nets and sjTaaptic systems is correct, there 

 ought to be found in the animal series evi- 

 dences of transitions from one type to the 

 other. Herrick (1915) has stated very 

 clearly the essential differences between 

 these two types in the declaration that in 

 nerve-nets there are no sjTiapses and no 

 polarity, both of which characterize the 

 more differentiated tj-pe. The many illus- 

 trations that have been used to show the 

 structure of nerve-nets from the ccelen- 

 terates to the vertebrates exhibit continu- 

 ous diffuse nets without the least sugges- 

 tion of sj'napses. Some of the best of these 

 examples are from the subumbrellar sur- 

 faces of jellj-fishes. Here, too, conduction 

 has been studied for a long time and it 

 has been shown through the work of 

 Romanes (1877) and others that transmis- 

 sion in these regions is as diffuse and gen- 



