igii.] 



THE PRBIITIVE NERVOUS SYSTEM. 



223 



As opposed to Hertwigs' hypothesis of the origin of nerve and 

 muscle, I wish to present certain facts obtained from a study of 

 sponges. As is well known, sponges are extremely primitive meta- 

 zoans, more primitive even than the ccelenterates. All attempts to 

 demonstrate in them sensory or other nervous structures have yielded 

 negative results, so that the majority of investigators of this group 

 have come to regard sponges as devoid of true nervous structures. 

 Not only are sponges without parts that can be reasonably called 

 nervous, but so far as I have been able to ascertain by an extended 

 study of a species of Stylotella, they show none of those qualities of 

 transmission and relatively quick reaction which characterize even 



KnacniL 



Fig. 2. Diagram to illustrate Hertwigs' theory of the origin of nerve 

 and muscle. A, epithelial stage: B. partially differentiated muscle-, nerve- 

 and ganglion-cell; C, muscle-, nerve- and ganglion-cell of ccelenterate stage. 



such animals as have only primitive nervous systems. In fact in 

 this respect sponges resemble plants rather than animals. But not- 

 withstanding the fact that sponges show no evidence either anatom- 

 ical or physiological of possessing nervous organs, they are not 

 without powers of response. Stylotella, for instance, can open and 

 close its oscula and its lateral pores, and can even contract its flesh 

 more or less. These movements, to be sure, are carried out ver\' 

 slowly, but they follow certain stimuli with such regularity that they 

 must be regarded as true responses. Thus the oscula of this sponge 

 close regularly when the water about these openings becomes quiet 

 and reopen after the water has been again set in motion. These 

 movements of the sponge are carried out by contractile tissue which 

 has the appearance of smooth muscle-fiber and which, like smooth 

 muscle, responds with great slowness. The slowness of the response 

 is so marked even in comparison with what is met with among the 



