Respiration in Invertebrate Animals. 195 



the Pulmonata should be desciibcd as the pericardium, re- 

 ceiving air instead of water into its interior. The vascular 

 system of Insects is filled with air, that of the Annelids with 

 fluid. Insects differ from the Annelids, therefore, precisely in 

 the same manner as the Pulmonata differ from other MoUusks. 

 But, notwithstanding the express provisions which are thus made 

 to introduce the external element into the recesses of the body 

 of Cephalopods, these MoUusks are furnished with branchial 

 organs more beautifully and elaborately constructed than those 

 of any other Invertebrated animal. 



The general anatomy of this class is well known. The author 

 therefore will at once proceed to state the results of his special 

 researches on the minute structure of the respiratory organs. 



Mechanism of the Branchial Chambers. 



The branchial chamber of the Cephalopod is a perfect hydraulic 

 mechanism. It is placed in advance of the viscera. It is en- 

 closed laterally by the mantle, the muscles of which have received 

 an express disposition with reference to the rhythmic respiratory 

 movements which it is designed to perform. Anteriorly the 

 chamber is provided with two valvular openings, the valves 

 being so arranged as to afford a ready entrance to the inspira- 

 tory water-current, and to prevent its reflux. The water thus 

 drawn or sucked into the breathing-chamber is drawn or sucked 

 also into the hollow axes of the gills (especially in Octopus) by 

 a diastolic movement of these organs which seems to be syn- 

 chronous with that of the mantle. Having freely permeated the 

 branchiae, the water is expelled through the funnel by an expira- 

 tory act in which the gills and the mantle contract simulta- 

 aeously. The rectum and the ducts of the generative system 

 terminate in this chamber at the base of the funnel. The ex- 

 piratory current thus conveys externally the excreta. In this 

 character, as is well known, the Cephalopods coincide with the 

 Gasteropods. 



The precise mode in which the w^er, during the respiratory 

 movements, traverses the branchiae, has only recently become 

 known to the author*. It does not enter, as he formerly supposed, 



* In a former paper (see Annals, ser. 2. vol. xvi. pi. 9) I introduced a 

 figure (lig. 7) illustrative of the manner in which I then believed the water 

 to pass throjigh the branchiaj in the act of respiration. Although it was 

 intended only to convey a general idea of the mechanism of the chamber 

 and its currents, yet as that figure stands, it may lead to error. The hollow 

 axes of the gills are not open, as there represented, at their posterior or 

 attached extremity, but conically closed, as they are at the anterior extre- 

 mity. It will be evident, in the text, that the water cannot therefore enter 

 the interior of the organ in one large stream, as there shown. 



13* 



