No. 494] 



ZOOLOGICAL PROGRESS 



1 25 



Although these cell layers as such retain their limits in 

 a most striking and clear way in the bodies of the higher 

 animals, they must not be thought of as independent or in 

 any sense isolated in their development. Their mutual 

 relations are often of a most intimate kind and in the 

 course of their development these relations tend rather to 

 become more firmly consolidated than the reverse. This 

 condition is well illustrated by the growth of the neuro- 

 muscular mechanism in animals, a growth that can be 

 traced step by step through the multicellular forms till it 

 finally shapes itself into the complex machinery whereby 

 the animal reflexes are carried out. 



The first step in this process is seen in the sponges, 

 which in some respects are the simplest of the multicel- 

 lular animals. As is well known, these animals are quite 

 as sluggishly responsive to stimulation as plants are. A 

 long and unsuccessful search for their nervous organs has 

 ended in the belief that they possess no such structures. 

 Some of their cells, particularly in the neighborhood of 

 their numerous pores, are elongated or even fibrous and 

 are apparently contractile enough to serve as a means of 

 closing these openings. It is highly probable that these 

 contractile cells, which may be regarded as primitive 

 muscle cells, are brought into action by some stimulus 

 directly applied to them, such as dissolved materials in 

 the sea-water, etc. We are so accustomed to think of 

 muscle as controlled by nerve that independent action of 

 this kind seems wholly anomalous. And yet it must not 

 be forgotten that muscle can be made to contract by the 

 direct application of a stimulus and that even in the 

 higher animals under natural conditions this may occur. 

 Steinach has shown that when bright light is thrown on 

 the eye of a fish or an amphibian the pupil will contract 

 even after all vestiges of nervous connections have been 

 destroyed, and he believes this to be due to the direct 

 stimulation of the muscle fibers by light. This case sup- 

 ports the opinion that in sponges the contractile tissue 

 responds to direct stimulation, and in my opinion the 



