CEPHALOrODA. 305 



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 gi'.ls in the 

 Anellida, in some species of which both organs are irrelatively 

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

 vegetative orgar.s assume a more concentrated form in the Molluscous 

 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 twenty tufted gills of the Phyllidia, or of the four gills of the 

 Nautilus, as compared with the two branchiae with their perfect cir- 

 culation in the Sepia, that plurality 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 Anellides, and even when they first appear as jointed limbs in 

 the Myriapoda: but as they become progressively perfected, varied, and 

 specialised, they are reduced to ten in Crustacea, to eight in Arachnida, 

 and to six in Insecta. We have just seen that the same law prevails in 

 the introduction of the analogous cephalic organs of locomotion and 

 prehension in the Mollusca. It is beautifully illustrated in the in- 

 troduction of the organ of vision into the animal kingdom. 



The numerous ganglions, nerves, and muscles, which the vegetative 

 succession of the segments of the body and their locomotive appen- 

 dages in the Articulata calls forth, have sometimes been adduced as 

 invalidating the claims of the Vertebrata to be regarded as of higher 

 or more complex organisation ; but when the law of irrelative repe- 

 tition is rightly understood, the multiplication of similar parts for the 

 repetition of the same actions is at once appreciated as essentially 

 the more simple, as well as the inferior condition to the assemblage of 

 less numerous parts in the same body with different offices, and \vith 

 prospective arrangements that enable them to combine their different 

 powers for definite ends. 



The lowest Invertebrata resemble locomotive cells : they propagate 

 by spontaneous fission and grow by assimilation ; sometimes they 

 exhibit their geometrically multiplied divisions to a certain extent 

 within a common capsule. The earliest phenomena in the develop- 



