642 LECTURE XXIV. 



Such sacs are developed in great numbers in the body of the Poly- 

 gastrian, but each sac performs the same share of the digestive 

 function, irrespective of the rest. The case is very different in the 

 ruminant animal, in which each of the four stomachs has its appro- 

 priate office, and all combine together to produce a more efficient 

 act of digestion. 



The organs of generation, the next essential parts of the mere 

 animal, when first definitely introduced with their characteristic 

 complications in the low organised Entozoa, illustrate more forcibly 

 the law of irrelative repetition. 



We trace the definite development of the heart and gills in the 

 Anellids, in some species of which both organs are irrelatively 

 repeated above a hundred times. And when these, like most of the 

 vecetative organs, assume a more concentrated form in the Melius- 

 cous series, we perceive in the structure and relations of the two 

 auricles of the bivalve as compared with the single auricle of the 

 univalve, and of the four gills of the Nautilus as compared with the 

 two gills with their more perfect circulation in the Sepia, that plu- 

 rality is but a sign of inferiority of condition. 



When locomotive and prehensile appendages first make their 

 appearance in free animals, they are simple, soft, and unjointed, but 

 they are developed by hundreds, as in the Asterias and Echinus : 

 they manifest the principle of vegetative repetition to a remarkable 

 extent when they are developed into symmetrical pairs of setigerous 

 tubercles in the Anellids, and even when they first appear as jointed 

 limbs in the Myriapods : but as they become progressively perfected, 

 vai'ied, and specialised, they are reduced to ten in Crustaceans, to 

 eight in Arachnids, and to six in Insects. We have just seen that 

 the same law prevails in the introduction of the analogous cephalic 

 organs of locomotion and prehension in the Mollusks. It is beauti- 

 fully illustrated in the introduction of the organ of vision into the 

 Animal Kingdom. 



Tlie numerous ganglions, nerves, and muscles, called forth by the 

 vegetative succession of the segments of the body and their locomotive 

 appendages in the Articulates, have sometimes been adduced as 

 invalidating the claims of the Mammalia to be regarded- as of higher 

 or more complex organisation*; but when the law of irrelative i*epe- 



* Thus the acute philosopher Young writes : — " There are even living beings, 

 visible to the microscope, of which a million million would not make up the bulk 

 of a common grain of sand. But it is still more remarkable, that, as far as we can 

 discovei-, many of these animalcules are as complicated in their structure as an ele- 

 phant and a whale. It is true that the physiology of the various classes of auiiuiils 

 is somewhat more simple as they deviate more from the form of quadrupeds and 

 fromi that of the human species ; and some of the lower classes appear to approxi- 



