22 IXTKODUCTIOX. 



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 Air- 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. Eightly 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 uterus, 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 testiculi) 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. 



