310 CIRCULATION OF NUTRITIVE FLUID. 



rials, and not to convey oxygen, this element being but little required 

 in the vegetative processes, and being supplied by other means, the 

 same energy and rapidity are not required in it, as need to be provided 

 for in the higher Animals. 



549. A condition of the Circulating system very similar to this, exists 

 in several of the lower animals, as well as in the embryo- state of the 

 higher. In the very lowest no blood-vessels are required, for the same 

 reason that no sap-vessels exist in the lowest Plants ; namely, because 

 every part absorbs and assimilates nutritious fluid for itself, so that it 

 does not require a supply from vessels. As, in the sea-weeds, the whole 

 substance is nourished by direct absorption from the fluid in contact 

 with the external surface, every part of which seems endowed with the 

 same absorbent power, so in Zoophytes do we find, that the whole sub- 

 stance is nourished by direct absorption from the internal surface, 

 which forms the lining of the digestive cavity. In the same manner, 

 the Aeration of the animal fluids, or the exposure of them to the air 

 contained in water, by which they may part with carbonic acid and 

 imbibe oxygen, is provided for, not by any special respiratory organs, 

 but by the contact of water with every part of the soft external and 

 internal surfaces. Further, as the substance of their body is nearly of 

 the same kind in every part, they do not require the continual inter- 

 change of the fluid distributed to its several portions. Thus no circu- 

 lation is necessary, in these simple animals, either for the nutrition of 

 their tissues, or for the aeration of the fluids. The same is the case 

 with others of the lower tribes ; as well as with the embryo of the higher 

 Animals, at the earliest periods of their development. Thus the lowest 

 Entozoa, or parasitic worms, have a digestive cavity channelled out, as 

 it were, in their soft gelatinous tissues ; and from the walls of this, the 

 nourishment is drawn by the several component parts of those tissues, 

 without the mediation of vessels. And the embryo even of Man, in its 

 early condition, consists of an aggregation of cells, each of which absorbs 

 for itself from the nutritious fluid with which it is surrounded, and goes 

 through all its functions independently of the rest. 



550. Proceeding a little higher, we find the first appearance of proper 

 vessels in the higher Entozoa, and in the Echinodermata. These vessels 

 take up the nutritive fluid from the walls of the digestive cavity, on 

 which they are spread out, just as the roots of Plants do from the soil. 

 They then unite into trunks, by which the fluid is conveyed to the more 

 distant parts of the structure, in the same manner as the ascending sap 

 is conveyed to the leaves by the vessels of the stem and branches ; and 

 these trunks again subdivide, and form a network of capillary vessels, 

 which are dispersed through the several parts of the fabric ; some of them 

 being very abundantly distributed upon a portion of the surface, which 

 is particularly destined to perform the respiratory function. Through 

 these capillary vessels, the fluid seems to move in very much the same 

 manner, as through the system of anastomosing vessels in Plants ; that 

 is, its motion is due, rather to forces which are developed during its 

 circulation, than to any vis a tergo derived from the contractile power 

 of a propelling organ. But there is this difference : that, after having 

 traversed the minute vessels, and yielded up to the tissues a part 



