640 LKCTURE XXIV. 



lowest of the Invertebrate animals could we have found the office of 

 the lung performed by a vascular portion of the integument ; and the 

 Invertebrata alone could have furnished us with examples of the 

 progressive modifications of this portion of the vascular skin, by 

 which the breathing organ is at length definitely developed. But 

 why specially cite the respiratory system ? The earliest and most 

 instructive forms of almost every organ, as it is variously 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 oscillation through 

 the living tissues of a fluid which seems to differ but little from sea- 

 water, and of the direct communication of the chylaqueous system 

 with the external oceanic medium; — of the co-existence of an aqui- 

 ferous with a sanguiferous system in the same animal, and the pro- 

 gressive development of a true vascular organisation by which the 

 aquiferous channels are at length superseded ; — of a circulation of 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; — of 

 the meandering of the circulating fluid back to the heart through 

 diffused venous lakes, instead of cylindrical canals. Only the low 

 organised Invertebrate animal could have revealed to us the actual 

 existence in nature of a condition of the blood's motion, once errone- 

 ously held to be universal, viz., its flux and reflux in the same 

 vessels, from trunks to branches and from branches to trunks, to and 

 from the heart *, as Aristotle conceived to be the nature of the blood's 

 movement in all animals, and which he illustrated by the analogy of 

 the tides of the Euripus ! 



In the Invertebrate series we can contemplate the most simple and 

 essential condition of the nervous system, — a ganglion with radiated 

 internuntiate chords, — a centre for the reception and transmission 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 con- 

 ditions for the performance of the most essential function of the ner- 

 vous 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.f 



* Lecture XX. Tunicata, p. 469. f Lecture XVL Insecta, p. 364. 



