Respiraiion in Invertebrate Animals. 407 



entirely suppressed, and the visceral cavity obliterated. This 

 vulgar worm, however, does not breathe on the atmospheric, but 

 on the aquatic principle. It dies rapidly in perfectly diy places. 

 Its cutaneous surface is the scene of a dense plexus of blood- 

 proper vessels. It is always enveloped in a stratum of viscid 

 fluid, which is remarkable for the property of absorbing and dis- 

 solving atmospheric air. This air, brought thus into immediate 

 and intimate contact with the surface of the body, operates 

 directly upon the blood-proper circulating in the cutaneous 

 plexus. In the Abranchiate Annelids as in many of the tubico- 

 lous Annelids, the alimentary canal is profusely supplied always 

 with a vascular tissue which sha-es in the respiratory process: 

 this process may be distinguished as the intestinal respiration. 



In the genus Clymene (fig. 9) the branchial organs are situated 

 at the tail (c). In ultimate structure they correspond in every 

 particular with those of the Sipunculidse — they are liollow mem- 

 branous projections (B) penetrated by the chylaqueous fluid [e) 

 in which a coiled blood-vessel {d) floats. They are destitute of 

 cilia. They afford the only illustration in the class Annelida 

 of branchial organs specialized around the outlet of the ali- 

 mentary system. 



It has now been shown that the branchial organs in the An- 

 nelida arrange themselves under two leading divisions, between 

 which a clearly legible line of demarcation exists. Under the 

 one, the blood-vessel-bearing branchise occur ; under the other, 

 those range which are organized for the exposure of the chyl- 

 aqueous fluid. Vibratile cilia are never superadded when the 

 blood-proper alone enters the gills ; generally, when this and the 

 chylaqueous fluid participate in the process ; always, when the 

 latter alone enacts this function. The mechanism of respiration 

 in the Annelid demonstrates beyond doubt that the agency of 

 floating corpuscles is not required for the absorption of the ex- 

 ternal oxygen. The blood-proper, though coloured, is non- 

 raorphotic in every species. It has been proved that the ten- 

 tacles are not simply organs of touch : with a single instrument 

 nature accomplishes various ends. They are subsidiary organs 

 of respiration. They are injected always with the chylaqueous 

 fluid. It is certain therefore that in the oeconomy of the Annelid 

 the blood-i)roper and chylaqueous fluid are co-equal elements ; 

 they are convertible proximate principles ; they exhibit equal phy- 

 siological capacities ; both are capable of discharging the func- 

 tion of respiration, and both are capable of supplying the solids of 

 the body with the materials of increase. 



