On the Mechanism of Aquatic Respiration. 243 



XXV. — On the Mechanism of Aquatic Respiration and on the 

 Structure of the Organs of Breathing in Invertebrate Animals. 

 By Thomas Williams, M.D. Lond., Licentiate of the Royal 

 College of Physicians, formerly Demonstrator on Structui'al 

 Anatomy at Guy's Hospital, and now of Swansea. 



[With a Plate.] 



The mechanism of breathing in the countless hosts of inverte- 

 brate animals which people the ocean, offers a problem which has 

 never yet been satisfactorily solved. The mode in which life is 

 sustained in those degraded forms, in which " a circulation of 

 blood " is not to be discovered, has long stimulated the curious 

 wonder of the naturalist. Fishes and Cetacea excepted, the in- 

 vertebrate animals constitute the entire population of the ocean. 

 Insects excepted, all invertebrate animals are aquatic. Hence 

 the wide range of interest which belongs to this subject. How 

 animals breathe is not second in importance to the question 

 how they live. Every observer studies the latter, few the former. 

 There are " habits " associated with the manner in which the 

 function of breathing is performed which are well-fitted to win 

 admiration. Wanting the knowledge of this process, not the 

 smaller half of the history of an animal remains to be acquired. 



It is the aim of this memoir to demonstrate first the ana- 

 tomical conditions under which the office of respiration is per- 

 formed in the invertebrate animals, and then to study the pro- 

 cess itself. The anatomical conditions will prove as various as 

 the classes of which this subkingdom is composed. Two pri- 

 mary divisions of this subject demand at once to be recognised ; 

 — 1st, that comprising those organs which adapt the animal for 

 atmospheric breathing; 2nd, that qualifying it to respire in 

 water. The latter, embracing varieties more striking and 

 numerous than the former di\'ision, should again be resolved into 

 two denominations, of which one would comprehend the me- 

 chanism of those organs by which the chijlaqueous fluid is sub- 

 mitted to the agency of the aerating element, and the other, 

 that of those fitted to expose the true blood*. 



All vertebrated animals, fishes excepted, breathe on the atmo- 

 spheric plan. All invertebrate animals, insects excepted, respire 

 on the aquatic model. The organs used in the first method are 

 more complex than those comprised in the second ; while the 

 chylaqueous fluid is subjected to respiration, through the least 

 complexly arranged mechanisms. The simpler the fluid to be 



* The author would here beg to refer the reader, for a full statement of 

 the grounds of this latter subdivision, to his paper on the Blood-proper 

 and Chylaqueous Fluid, &c., in the Phil. Trans., Nov. 1852. 



17* 



