Respiration in Invertebrate Animals. 45 



observes, " They do not appear to pass into a defined system of 

 vessels, .... but find their way through the interstices of the 

 various organs in the various cavities of the body. . . . They pro- 

 ceed by jerks ; some find their way into the space between the 

 breathing surfaces, and slip in between the rows of oral rings 

 {stigmata), and wind along down between the rings in irregular 

 courses*." 



In the Ascidiadas and Clavellinidse the centres of this system 

 consist of two trunks, a dorsal and a ventral, the capillary 

 system of the branchiae being intermediate. Lister's famed ob- 

 servations on this subject should be consulted f. The descrip- 

 tive details afterwards to be presented on the subject of the 

 respiratory and circulatory systems of the Acephalans, will illus- 

 trate many points of interest in the structure of the correspond- 

 ing systems of the Tunicata. The peripheric channels of the 

 blood are analogous in the two classes. The ultimate structure, 

 though not the arrangement of the vessels of the branchiae, is 

 also similar. The nutritive fluids, morphotically distinctive,- are 

 chemically identical in the two classes. 



Acephala. 



In the Terebratulidse there exists no express apparatus for 

 breathing. With the Craniadse they are therefore placed at the 

 inferior limit of the Lamellibranchiate series. Prof. Owen has 

 shown that the -mantle in the Brachiopods is more vascular than 

 in those orders of bivalves in which gills txist. Dr. Carpenter J 

 has lately shown that the external layer of the mantle in Tere- 

 bratula and certain other Brachiopoda, sends out ccecal tubes 

 through the shell. They are respiratory in office, and the exact 

 counterpart of those membranous processes which the author of 

 these papers has described in the Echinodermata as projecting 

 up above the external surface of the body. The ccecal character 

 of these parts establishes a community of type between the fluid 

 system of the Brachiopods and the chylaqueous system as de- 

 fined by the author. The arms are long, richly ciliated tubes. 

 In these tubes the blood moves in a single channel by flux and 

 reflux. This incident also in the history of the fluids allies 

 these inferior mollusks with those animals in which a chylaqueous 

 system only exists. This latter fluid never undergoes an orbital 

 movement : it fluctuates to and fro. The ultimate respects in 

 which the vessels in the mantle of the Brachiopods differ from 



* See his interesting work, ' A Naturalist's Rambles on the Devonshire 

 Coast,' p. 245. 



t Phil. Trans. 1834. 



X Proceedings of the Roval Society, April 6, 1854. 



