4 PHYSIOLOGY OF THE INVERTEBRATA. 



excretes, and reproduces — for no special part is set aside for 

 the functions of digestion, of respiration, of excretion, of re- 

 production. There seems to be in the lowest Invertebrates a 

 confusion of organic materials and functions. Many of the 

 Protozoa are endowed with motility and sensibility, with a 

 sort of instinct ; * and yet, as far as we know at present, they 

 are destitute of muscular and nervous elements. Possibly 

 the sarcode is the rudiment, still undivided, of muscular 

 fibre. 



But as we ascend gradually from lower to higher forms 

 the differentiation becomes more marked, and we find par- 

 ticular parts of the body reserved for special actions. But 

 this differentiation passes through various stages before 

 arriving at the most differentiated forms of animal life. As 

 already stated, the single cell of the amoeba performs many 

 functions ; and even when an organ has arrived at such a 

 stage that it is quite distinct, it may have a dual or triple 

 function — as, for instance, the pentagonal pyloric sac of 

 Uraster ruhcus (one of the Asteridca) has been proved to 

 have a dual function. f It is a digestive gland as well as an 

 excretory organ, separating the nitrogenous products of the 

 waste of the tissues, &c., from the blood in the form of uric 

 acid, which is to be found in the five pouches of that 

 organ. In The Orif/in of Sjjccies (cha,])tev vi.) Darwin mentions 

 the fact that " numerous cases could be given among the 

 lower animals of the same organ performing at the same 

 time wholly distinct functions : thus, in the larva of the 

 dragon-fly .... the alimentary canal respires, digests, and 

 excretes." But as we pass from the lower to the higher 

 forms of animal life the various organs have special functions 

 assigned them. This rule not only applies to the physio- 

 logical functions of various organs, but also to their ana- 



* See Binet's Psijcliic Life of Micro-Organisms. 



+ See Dr. A. B. Griflith.s' papers in the Proceedings of Royal Societg of 

 London, vol. 44, p. 325 ; and the Proceedings of llogal Society of Edinhurghy 



vol. 15, p. III. 



