i2 4 TEE POPULAR SCIENCE MONTHLY 



protoplasmic transmission, a forerunner of the true nervous impulse, 

 and as such gives us some insight into the nature of the non-nervous 

 transmission in sponges. The results of my studies on Stylotella support 

 the conclusions of most biologists who have worked upon sponges, that 

 these animals probably possess no true nervous tissue. Their muscles, 

 in my opinion, are brought into action almost entirely by the direct 

 effect of the stimulus rather than through nerves, and this accounts, I 

 believe, for their very slow response to external disturbances. It is 

 possible that in certain sponges some form of nervous tissue may be 

 demonstrated eventually, or that such organs as those described by von 

 Lendenfeldt as synocils may be shown to have a sensory significance, 

 but such cases, if they do occur, will probably remain exceptional, for 

 as a whole sponges seem to be a group of animals almost if not 

 quite devoid of true nervous tissue. Granting this conclusion, it must 

 be evident that the condition in sponges throws a very important 

 light on the question of the origin of nerve and muscle. Their state 

 suggests at once that nerve and muscle have not been differentiated 

 simultaneously, as maintained by Kleinenberg, the Hertwigs, and 

 others, but that muscle preceded nerve in its evolution and that sponges 

 represent animals with effectors but without differentiated receptors. 

 If then it may be claimed that phylogenetically the sense organ preceded 

 the central nervous organ, it may also be maintained that muscles pre- 

 ceded sense organs. Thus the three elements of the reflex arc of the 

 higher animals were probably evolved separately and in the order, 

 effector, receptor, adjustor. 



If muscle originated before nerve and was brought into action at 

 first by direct stimulation, it is natural to expect that examples of this 

 form of response might still be found among the higher animals. And 

 such seems to be the case. Thus the sphincter of the iris in the lower 

 vertebrates, though well known to be under the influence of nerves, was 

 shown by Steinach some time ago to be directly stimulated by light, a 

 condition which, judging from the more recent work of Hertel, prob- 

 ably applies even to the human eye. This muscle then exhibits a cer- 

 tain capacity for normal direct stimulation. Another example of the 

 same kind is seen in the embryonic vertebrate heart. Though the beat 

 of the adult heart may be a matter of controversy from the standpoint 

 of the myogenic and neurogenic theories, there can be no doubt that the 

 muscle of the embryonic heart beats, as shown by His, before it has be- 

 come invaded by nerves. And this view is supported by Barrow's recent 

 discovery that the isolated cells of the heart-muscle will contract 

 rhythmically under conditions where not the least vestige of a nerve can 

 influence them. Thus the embryonic heart-muscle and the sphincter of 

 the iris are muscles whose activity may be normally called forth by direct 

 stimulation, a condition which reproduces, so far as independence is con- 



