22 INTRODUCTION. 
Plates, Leaves, Threads, Twigs, &c. Many simple and imperfect 
animals breathe by means of the skin. Others, which have either 
no circulation, or none that is perceptible, have Adr-Canals, i.e. 
such respiratory organs as convey the air through the entire body 
to the nutrient fluid. : 
The nutrient fluid which has thus been separated from the food 
and changed by means of respiration, is now fit for the nutrition 
of the parts. How that nutrition is effected, so that every part 
receives from the common fluid that which is requisite for its 
support, is not known. Here we can only conjecture: and if any 
one chooses to call it a chemical affinity he is at liberty to do so, 
if he merely means that he is contemplating living creatures whose 
organism has a determinate chemical composition, and so does not 
forget that he has given a name to the process, but has not ex- 
plained it. 
Besides the glands which separate from the blood fluids for the 
internal economy, as the Liver, &c., there are others which separate 
constituents that must quit the blood in order that it may become 
more pure, or in order that the due proportion of its constituents 
may be preserved. Thus the kidneys secrete urine, the skin watery 
vapour, &c. Sometimes a secretion is a means of defence, as is the 
case with the Ink of the Cuttle-fish, and with the offensive exhala- 
tions of many animals, which thus repel their enemies or are avoided 
by them. Rightly to estimate all these secretions we must never 
forget that an animal is a whole, and that the secretion of this or 
that fluid, though it may be performed by an individual organ, is 
still under the control of all the other organs, and of life, which 
combines them all. 
Propagation, which also belongs to the vegetative life, has the 
following organs for its instruments: the ovary (ovarium), by which 
we understand the site and the coverings of the eggs and the eggs 
themselves, conjointly ; the oviduct (oviductus) or the tube, through 
which the eggs, that have been detached from the ovary, pass 
onwards: the wferus, a residence for the eggs during their develop- 
ment, and the vagina along which they pass to leave the body of 
the mother. In the case of two sexes, the male (by means of 
glands named festicul/) secretes the seed (sperma) which fertilizes 
the germs, and effects their development. Penis is the name of the 
part, which, in some animals, conducts the seed into the vagina of 
the female. 
