202 REPORT — 1851. 



is accomplished in the Abranchiate Annelids. Of this division of worms, 

 it is stated in all systematic works that the function of respiration de- 

 volves on " the external cutaneous surface of the body," and this is re- 

 garded as expressing the principle on which the same function is performed 

 in all Entozoa. It will be afterwards proved that between some »[)ecies 

 of Abranchiate Annelids and some species of Entozoa there really does 

 obtain almost an identity of structure, under a striking diversity of ex- 

 ternal form. In the Entozoa, however, the space between the peritoneum 

 and integuments is much larger than in the corresponding species of 

 Abranchiate Annelids. The difference affects materially the mechanism in 

 the two cases of the respiratory process. The Entozoa are remarkable for 

 the large amount of the peritoneal fluid ; the true blood-system being in 

 proportionate abeyance. In the Abranchiate Annelids, the system of the 

 peritoneal fluid is suppressed proportionately to the greater development of 

 that of the blood-proper. Here, as in other Annelids, the proportion 

 between the system of the chyl-aqueous fluid and that of the true-blood is 

 observed to be inverse. 



It may be affirmed as a law of the organization in all abranchiate worms, 

 that the system of the blood-proper is more developed on the parietes of the 

 intestinal canal than on the integuments. This fact, wherever the peritoneal 

 space is obliterated by the adherence of the intestinal cylinder to that of the 

 integument, transfers the office of respiration from the latter to the former 

 region ; that is, as is practically demonstrable in the instance of Naisjiliformis, 

 the large volume of water which is incessantly streaming throughout the 

 length of the alimentary canal, holding atmospheric air in solution, while it 

 ministers by its organic particles to the nutrition of the system, contributes 

 also by the air with which it is mixed, to the great purpose of aerating the 

 living fluids of the organism. This is accomplished partly by the exosmose 

 of the dissolved air from the m^ra-intestinal into the peritoneal or extra-in- 

 testinal space ; and partly by the absorption of it into the true-blood circu- 

 lating in the vascular plexus by which the intestinal parietes are embraced. 

 In the Entozoa it is not improbable that more of the aerating element is 

 derived by the peritoneal fluid ab extra, from the medium on which they are 

 parasitic, than ab intra from that which they swallow. Whichever of these 

 two great systems of the body (intestinal and integumentary) be entitled to 

 the o-reater share in the process of respiration in the Abranchiate Annelids, 

 it should be remembered that in the Entozoa the structure of the integuments 

 is almost entirely destitute of true-blood-vessels. This fact renders the infer- 

 ence probable, that the partition of the integuments, which in the Entozoa is 

 thin, is permeated by the gaseous elements without, and that they thus ewfer, 

 without meeting much of the true-blood in transitu, into the fluid of the 

 visceral cavity, by which it is brought into contact with the solids of the body. 



In the genera Lumbricus and Hirudo, the peritoneal fluid and space being 

 very small, the function of breathing falls on the united structure of the 

 intestine and integument. These two genera are remarkable for the great 

 development of the reproductive organs, which occupy, to crowding, the inter- 

 val of the peritoneal cavity ; and for the excessive elaboration of the system 

 of the blood-proper. 



The genus Trophonia is characterized, as compared by the former, by an 

 increase in the volume of the contents of the peritoneal fluid. Here also all 

 , traces of external branchiaj are wanting; life is maintained through the 

 aerating influence of the chyl-aqueous fluid. 



In the Naides, observation proves beyond doubt, that breathing is accom- 

 plished through the medium of the peritoneal fluid; its movements are 



