An Introduction to Zoology. 31 



vessels ; — we say supposed, for it is more than probable that our know- 

 ledge of the internal structure of the lower zoophytes is but in its infancy. 

 With regard to the infusoriais, or animals of infusions, for the most part 

 microscopic, the researches of M. Ehrenberg have shewn that their organi- 

 zation was far more elaborate than ho.d been supposed ; and in them he 

 has discovered not only stomachs and an alimentary canal, but what he 

 considers as analogous to the liver and pancreas. But we shall return to 

 this hereafter. 



Advancing a step higher, we find in insects and the testaceous or shell- 

 inhabiting raollusca, a regular hepatic system for the secretion of bile. 

 Now the chyle thus prepared is next to be absorbed into the system. 

 In all the vertebrated animals, the great absorbent system is divided into 

 two parts, and the branches of that peculiar one called the lacteal, are 

 distributed along the voluminous convolutions of the intestine, in order to 

 the taking up of the chyle; but in insects, mere transudation into the 

 pores of the general system seems to be the first step, although the rudi- 

 ments of a vascular system of circulation are even here partially to be 

 traced out. In the testaceous moUusca, (although in their organs of sense 

 inferior to insects,) the whole system of the circulation exhibits a nearer 

 approach to that of the higher classes, though the ordinary veins seem to 

 discharge the functions of the absorbent, which are therefore not found to 

 exist as a distinct system. Where this system, these lacteals, are present, 

 their ramifications at length uniting in one trunk, pour the chyle into the 

 great vein of the system, and thus bring it up to that step of the circula- 

 tion whicli is immediately preparatory to its being submitted, in its further 

 course, to the process of aeration. 



Aeration is absolutely essential to life, all forms of which, even the verv 

 lowest, would without it become extinct. Nor are even aquatic animals 

 an exception to this, though apparently inspiring a difterent element ; for 

 it is the atmospheric air intermingled with the water which they breathe; 

 and if this air be by any means abstracted from the water, the animals 

 inhabiting it will die. Even in ponds long frozen over, fish cannot support 

 existence, and will be seen flocking round any hole which may have been 

 broken in the ice, while life still remains. But although aeration is neces- 

 sary for the support of all life, it is an end accomplished in different orders 

 of animals by very different means. 



In the lower zoophytes, (which are all aquatic,) the nutritive fluid, in 

 generally percolating the pores of the system, seems to require nothing 

 more tiian the mere contact of the aerated water, along their general ex- 

 ternal surface. But in insects, tliough the nutritive fluid is equally difl'used, 

 a far more elaborate system of aeration is provided ; their bodies present 

 ranges of numerous circular orifices, each opening into the tube of a 

 trachaea, which branching out into multiplied ramifications, thus pervades 

 the whole mass. It has been well said, with reference to this structure. 



