288 PHYSIOLOGY OF THE INVERTEBRATA. 



Astacus, and consequently the secretion of urine would be 

 most probably influenced. At any rate, this ia a question 

 for research. In the Vertebrata there is no doubt that the 

 nervous centres do greatly modify the secretions. Forty-six 

 years ago, Schiff demonstrated that lesions of the cerebral 

 peduncles rendered the urine albuminous and acid. Claude 

 Bernard* proved that punctures of the roof of the fourth 

 cerebral ventricle gave rise to the formation of glucose sugar 

 in the urine. Lesions of the isthmus and of the lower part 

 of the cervical marrow can prevent the urinary excretion,' or 

 in other words, produce anuria. 



There is no doubt that in the Invertehrata the nerves play 

 an important part in the phenomena of secretion, and 

 even in the lower orders, where there are no traces of nervous 

 elements, the protoplasm of the cells, being irritable, is 

 capable of bringing, into play the phenomena which we have 

 been discussing in the present chapter. Experimental evi- 

 dence shows that the Amceha, for instance, excretes, digests, 

 and respires; but so far at least as present microscopic 

 expedients reach, this organism appears to be simply a small 

 mass of protoplasm, nevertheless it has the power of adjust- 

 ing its low organisation to the environment. "In the 

 organism lies the principle of life ; in the environment are 

 the conditions of life. Without the fulfilment of these con- 

 ditions, which are wholly supplied by environment, there can 

 be no life." 



The wonderful adaptations of each organism, and of each 

 part of every organism to its environment, inspire us with a 

 sense of the boundless resource and skill of Nature in perfect- 

 ing her arrangements for each single life. The causes of 

 these adaptations are to be sought in the numberless structural 

 modifications brought about by means of natural selection 

 and by the direct action of the environment. 



As already stated, not only an organism as a whole, but 

 each organ is also capable of undergoing modification. Hence 

 * Legons de Physiologie Op6ratoire. 



