CEPHALOPODA. 363 



But why do I cite the respiratory system ? The earliest and most 

 instructive forms of almost every organ, as it is permanently arrested 

 in its development and adapted to the exigencies of the mature 

 animal, must be sought for amongst the Invertebrata. « 



These animals alone furnish us examples of the circulation of the 

 blood without a heart, and of a circulation aided by the hundred- 

 fold repetition of the propelling reservoir in the same individual. In 

 these alone we meet with the singular condition of the circulating 

 fluid meandering back to the heart through diffused venous lakes, 

 instead of cylindrical canals. Only the lovv organised Invertebrate 

 animal could have revealed to us the actual existence in nature of a 

 condition of the blood's motion, once erroneously held to be uni- 

 versal, viz. its flux and reflux in the same vessels, from trunks to 

 branches and from branches to trunks, to and from the heart.* 

 Could this remarkable exceptional case of the minute Ascidian have 

 been shown to the antagonist of Harvey, how triumphantly he 

 might have appealed to the evidence of the senses as demonstrative 

 of the doctrine which Aristotle had taught and illustrated by the 

 analogy of the tides of Euripus ! 



In the Invertebrate series we can contemplate the most simple and 

 essential condition of the nervous system, — a ganglion with ra- 

 diated internuntiate chords, a centre for the reception and trans- 

 mission of stimuli. In this series we see such centres multiplied and 

 set apart for different segments, or for different organs of the body. 

 Of such arrangements, the anatomist of the Vertebrata exclusively 

 could have formed no adequate conception. And not only are the 

 conditions for the performance of the most essential function of the 

 nervous system, — viz. the working of the muscular machinery agreeably 

 with impressions received from external objects, independently of 

 any consciousness of such changes, or of any choice and volition in 

 the mode of remedying or avoiding them, — most plainly set forth in 

 the Invertebrate types of the nervous system ; but they afford the 

 best subjects for illustrating that primary function by experiment. 

 No vertebrate animal, for example, could have yielded such striking 

 results as have been detailed in the experiments on the Mantis, t 



The Invertebrate animals reveal the constant relation of the brain 

 to a position above or behind the alimentary tract, and demonstrate 

 that organs of special sense govern the existence of the supra- 

 cesophageal nervous mass, and that its increase of bulk is in the direct 

 ratio of their increase in number or complexity. 



The relation of the physical defence of an animal by shells or 



* Lecture XX. TuNicATA, p. 27;?. f Lecture XVL Insecta, p. 207. 



