ASSIMILATING GLANDS—ABSORPTION IN INVERTEBRATA. 201 
_ 224. There are certain glandular bodies, disposed in various 
parts of the system, which seem to discharge a similar office ; 
withdrawing the raw material (so to speak) from the general 
_ eurrent of the circulation, and returning it again in a state of 
_ higher elaboration. Such are the Spleen, the Thyroid and 
Thymus glands, and the Supra-Renal capsules. Besides these, 
the Liver probably exerts an assimilating action upon the crude 
materials which are made to pass through its substance, almost 
immediately after having been received into the blood-current, 
and before they are allowed to pass into the general circula- 
tion ; the whole of the blood returned by the gastric and 
mesenteric veins from the walls of the alimentary canal, being 
conveyed through the liver by the portal system, in its way to 
the heart (§ 267). 
225. In the Invertebrated animals, neither lacteals nor 
lymphatics exist; and the blood-vessels, whose absorbent 
— are to a certain extent restricted in the higher animals, 
have to perform the functions of these. There are animals, 
however, which are destitute not only of lacteal and lymphatic 
vessels, but even of blood-vessels ; and in these, as in the 
Cellular Plants, there is but little transmission of fluid from one 
_ part of the body to the other; for every portion, both of the 
internal surface (or lining of the stomach), and of the external 
surface which is bathed in the surrounding fluid (for most of 
these animals are aquatic), seems equally to possess the power 
of absorption ; and the parts to whose nourishment the fiuid 
thus received into the body is to be appropriated, are in the 
immediate neighbourhood of those which have absorbed it. , 
This is the case, for example, in the Hydra and Sea-Anemone, 
and, more or less, in all the Polypes ; as well as in the lower 
Worms. Between these, therefore, and the Cellular Plants, a 
remarkable analogy exists in regard to the mode in which the 
nutriment is absorbed and applied ; the difference being, that 
the Animal possesses a digestive cavity, lined by an inward 
extension of the external surface, which does not exist in 
Plants (§ 8). And it is upon the walls of this cavity, that 
_the absorbent vessels of the higher Animals (whether lacteals 
or blood-vessels) are distributed, collecting the nourishment 
im contact with them ; just as the roots of a Plant, spread 
‘through the soil, draw up that which it contains. But among 
those lowest animals in which the digestive cavity altogether 
