698 SPECIAL PHYSIOLOGY. 



Annuloida. In these animals, nothing analogous to a blood system exists. 

 there being neither blood nor true bloodvessels, or lacunar blood spaces. The 

 wattr-vessels of the Roti;era or Wheel-animalcules, of the marine Turbellarian 

 Worms, the Trematode Flukes, the Nematode Thread-worms, and the tape- 

 like Tsenia and its allies, the Acanthocephali, are ciliated canals, which com- 

 municate, at some point, with the exterior, and are, by some, regarded rather 

 as respiratory organs. Neither a heart nor a dorsal vessel has been found in 

 them. The extensive system of ambulacra! and other vessels, possessed by 

 the Echinodermata, also communicates with the exterior, and is, apparently, 

 partly respiratory, and partly locomotive. No heart or bloodvessels have been 

 detected in these animals. 



Codenterata. These are also destitute of blood, cardiac apparatus, and 

 proper circulatory vessels. In the Actinozoa and Hydrozoa, the digestive 

 cavity communicates with the perivisceral or general cavity of the body, from 

 which ramifications, sometimes very fine and numerous, proceed through .the 

 disc and other more developed parts. In this way nutritive fluids get access 

 to all parts of the body, without a circulation of blood. 



Protozoa. The singular, characteristic, slowly pulsating vesicles, which 

 alternately contract and dilate within the soft sarcodous substance of the In- 

 fusoria and Rhizopoda, although they sometimes present radiating ramifica- 

 tions, as in certain Infusoria, are probably respiratory rather than circulatory. 

 In the sarcode of the sponges, and in the unicellular Gregarinida, there are 

 not even contractile vesicles ; and no trace whatever of internal vessels. The 

 circulation of external fluid through the porous substance of a sponge, has, 

 of course, no affinity to a true internal circulation. 



NUTRITION. 



The general process by which the microscopic elements of the tis- 

 sues and organs of the body are maintained in a healthy condition, 

 and are renovated after disintegration and waste in use, constitutes the 

 function of Nutrition. 



Nutrition proper, consists in the maintenance of a living part, with- 

 out any important change of form or size. Growth implies a more 

 active nutritive process, resulting in an increase of dimensions. De- 

 velopment, besides nutrition and growth, requires evolution, or changes 

 of form. Reparation is a nutritive act, which has for its object, the 

 healing of a wound or loss of substance, or the more .perfect restora- 

 tion of an injured or lost part. Reproduction of a lost part, is an 

 extreme example of reparation. The act of secretion is a nutritive 

 process, in which the products of nutrition are incessantly thrown out 

 from the system. The reproduction of the individual, is a special nu- 

 tritive phenomenon. 



In Man and the higher animals, the blood is the common source of 

 the nutrient material for the solid parts of the frame, and, in its rapid 

 circulation through the body it passes, whilst in the capillaries, within 

 a very minute distance of the elementary constituents of the various 

 tissue's and organs. In yielding this nutrient material, the blood itself 

 is necessarily impoverished, and accordingly is, in its turn, renewed 

 or nourished, chiefly from the lymph and chyle, both of which are like- 

 wise constantly being reformed or renewed. All three fluids, the 

 lymph, the chyle, and the blood are, however, more or less organized, 



