2 PHYSIOLOGY OF THE INVERTEBRATA. 



matter, and depends upon one's definition of a true kidney. 

 Emhryologically these vesicles are the remains of a part of the 

 original colon. 



As more attention has been paid to the morphology of the 

 Invertebrata, it is not our object to speak of that branch of 

 the subject further than is necessary ; but in some cases the 

 function of an organ or a tissue cannot be comprehended 

 without referring to its anatomy. 



According to the great apostle* of biological thought, " the 

 actions of living matter are termed its functions ; and these 

 functions, varied as they are, may be reduced to three 

 categories. They are either — (i) Functions which affect the 

 material composition of the body, and determine its mass, 

 which is the balance of the processes of waste on the one 

 hand, and those of assimilation on the other. Or (2) they 

 are functions which subserve the process of reproduction, 

 which is essentially the detachment of a part endowed with 

 the power of developing into an independent whole. Or (3) 

 they are functions in virtue of which one part of the body is 

 able to exert a direct influence on another, and the body, by 

 its parts or as a whole, becomes a source of molar motion. 

 The first may be termed sustentativc, the second generative, 

 and the third correlative functions. In the lowest forms of 

 life the functions which have been enumerated are seen in 

 their simplest forms, and they are exerted indifierently, or 

 nearly so, by all parts of the protoplasmic body ; and the like 

 is true of the functions of the body of even the highest 

 organisms, so long as they are in the condition of the 

 nucleated cell, which constitutes the starting-point of their 

 development. But the first process in the development is 

 the division of tfie germ into a number of morphological 

 units or blastomeres, which eventually give rise to cells ; and 

 as each of these possesses the same physiological functions 

 as the germ itself, it follows that each morphological 



* Prof. Huxley. 



