362 LECTURE XXIV. 



tioiis of the Invertebrate animals, by the busy care-worn Surgeon 

 who, in the last century, j^repared the most instructive illustrations of 

 our present lectures, were not understood, and that these occupations 

 of Hunter were regarded by some of his contemporaries as a kind 

 of laborious trifling. But the question of " cui bono ? " which might 

 then be pardonable, is surely inexcusable at the present day, when the 

 relations of Surgery as a science to Physiology, and the dependence of 

 Physiology upon Comparative Anatomy ought to be better understood. 

 I would not, however, desire a more satisfactory answer to any re- 

 maining scepticism as to the necessity or importance of the dissections 

 of the lower organised animals, than your reception of the present 

 course of lectures ; and I trust that a very brief retrospect of some of 

 the deductions that have been obtained from the details to which you 

 have patiently listened, will suffice to demonstrate their value to all 

 who are interested in the progress of physiological science. 



The Invertebrated classes include the most numerous and diversified 

 forms of the Animal Kingdom. At the very beginning of our 

 inquiries into their physiology we are impressed with their important 

 relations to the maintenance of life and organisation on this planet, 

 and their influence in altering and augmenting the crust of the earth 

 itself, relations of which the physiologist conversant only with the 

 Vertebrated animals must have remained ignorant. At our first 

 entrance, and by the lowest portal, into the vast and intricate repo- 

 sitories of the animal mechanisms, we are at once introduced to the phe- 

 nomena of spontaneous fission and ciliary motion, of the generality and 

 importance of which in the animal economy each day seems to bring 

 fresh proof, but which are most conspicuously manifested in the Monads. 



The physiologist must have remained in ignorance of the most 

 instructive modifications and combinations of the principal organs of 

 the animal frame, if the researches of the Comparative Anatomist had 

 been confined to the Vertebrated animals, or to those which are con- 

 structed on the same general type as ourselves. Without a much 

 deeper investigation, we never could have understood the relative 

 importance of the diff'erent organs, or have become acquainted with 

 their most striking analogies. 



Only in the Invertebrated classes do we find examples in which 

 the lungs, like the liver, are supplied by the ramifications of the 

 trunk of a vein without the interposition of a heart. Only in the 

 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. 



