140 



ANIMAL. 



With regard to structure, as may be imagined, 

 the amorphous tribes, at the bottom of the 

 scale, are the most simple of all. The bodies 

 of some of these are without any internal 

 cavity, and without any division of parts; 

 they are homogeneous masses, generally gela- 

 tinous in appearance, and simply cellular in 

 structure, without arrangement into tissues or 

 particular organs. The external surface of 

 these animals imbibes the matters which are 

 fitted to subserve the purposes of nutrition, 

 and we may presume that it throws off by 

 transpiration such particles as are worn out 

 or have accomplished this end. The external 

 surface is also the organ of respiration in these 

 animals. They procreate by the evolution of 

 gemmi from their surface, and if they possess 

 sensibility the element to which it is attached 

 must be generally diffused throughout their sub- 

 stance. 



The organization of the radiata becomes con- 

 siderably more complicated. Fluids are no 

 longer absorbed from the external surface of 

 the body ; we meet with an internal cavity, the 

 rudiment of a digestive apparatus, having a 

 single opening in some of the species, which 

 serves consequently for both mouth and anus, 

 but in others presenting two openings, a mouth 

 properly so called on one side of the body, and 

 an anus on the other. Through the walls of 

 this cavity the nutritive fluids make their way, 

 and infiltrate the general mass of the animal's 

 body. In this class we also discover the 

 rudiments of a nervous and of a muscular sys- 

 tem. The nervous system consists of rounded 

 masses of a soft whitish substance, equal in 

 number to that of the radii composing the 

 animal, connected together by slender white 

 cords, and sending off filaments of the same 

 description to all parts of the body, but espe- 

 cially to the outer integument, and to the inter- 

 nal digestive apparatus. The muscular system 

 consists of reddish and whitish fasciculated 

 fibres disposed in the line of the motions. The 

 external surface of these animals is still the only 

 organ of respiration they possess. 



The three systems now enumerated the 

 digestive, the nervous, and the muscular are 

 readily demonstrated in the majority of the 

 symmetrical animals, and are even very soon 

 found to have acquired complication, and to 

 have sundry other parts and organs superadded 

 to them. The digestive apparatus consists of 

 .a mouth for the susception of aliment, of a 

 stomach for its elaboration, of an intestinal 

 canal from which the nutrient juices are ab- 

 sorbed, and of an anus from which the un- 

 digested residue is expelled. Whilst in the 

 radiata the nutritious fluids passed through the 

 parietes of the digestive cavity to impregnate 

 the body of the animal, and be assimilated 

 with its substance ; in the binaria we find 

 vessels, the mdiments of a circulating system, 

 employed in receiving the juices prepared in 

 the digestive apparatus and transmitting these 

 to all parts of the body. Digestion, too, in this 

 class becomes a more complicated process than 

 in the radiata, and various secreted fluids, 

 WHY* and particularly bile, the special products 



of large and evidently important organs, are 

 added to the alimentary mass in its progress 

 through the intestinal canal. 



In addition to the digestive apparatus and 

 general exteinal respiratory surface we by-and- 

 by find an especial system dedicated to the 

 aeration of the juices prepared for nutrition ; 

 this is the respirator'/ apparatus. Of extreme 

 simplicity in the first instance, being little 

 or no more than a fold of integument turned 

 inwards, and forming a simple cavity or sac 

 within the body of the animal, it is soon 

 rendered more complex in its structure, being 

 distributed in the manner of vessels under the 

 name of trachea or canals to different parts of 

 the body, or being confined to a particular 

 district, and entitled lungs or gills as it is fitted 

 to receive the atmospheric air immediately, or 

 1o make use of this elastic fluid suspended or 

 dissolved in water. 



The existence of this separate respiratory 

 apparatus presupposes that of another system, 

 namely, the circulatory. The fluids prepared 

 by the organs of digestion are not yet fitted to 

 minister to the growth and nutrition of the 

 organization; to be made apt for this purpose 

 they require exposure to the air in the lungs or 

 gills wherever these organs exist, and these be- 

 ing distinct, or contained in a particular region 

 of the body, a series of conduits were re- 

 quired, first to carry the fluids thither, and 

 to transmit them subsequently to every part of 

 the organization for its support. Like all the 

 other systems of animals, the circulatory exists 

 of various degrees of complexness; when first 

 encountered it consists of a series of simple 

 canals or vessels, which diverge on every hand ; 

 by-and-by it has several, and finally one, forc- 

 ing piece, or heart superadded to it, which 

 impels the fluids by its contractions to every 

 the most remote part of the organization. 



Among animals, however, nutrition is not a 

 process simply of addition or composition ; 

 it is also, perhaps universally, one of subtrac- 

 tion or of decomposition. We have seen the 

 composition provided for by special systems 

 in animals occupying very low grades in the 

 scale of creation ; we mount but a short way 

 before we encounter an apparatus which pre- 

 sides over the decomposition also in the shape 

 of another system of vessels, the veins and 

 especially the lymphatics ; these collect the 

 superfluous and worn-out particles from every 

 part, pour them into the general current of the 

 circulation, wherein being exposed in the vital 

 elaboratory of the lungs they are either assi- 

 milated anew and made fit once more to form 

 an integral part of the organization, or, being 

 subjected to the action of certain glands, they 

 are singled out, abstracted, and finally ejected 

 from the system entirely. In the most com- 

 plicated animals therefore a peculiar appa- 

 ratus for the depuration of the system is su- 

 peradded as complementary to the absorbents. 

 This we find in the glandular bodies familiarly 

 known as the kidneys ; the vehicle in which the 

 decayed particles are withdrawn is the urine. 



\\ hen we examine the instruments of sensa- 

 tion, we find them becoming gradually more 



