402 Dr. T. Williams on the Mechanism of Aquatic 



seen, when viewed by transmitted light, coils of naked blood-ves- 

 sels ; in those instances in which the branchiae are laminated or 

 foliaceous, as in Nereis renalis, the blood-plexus on the external 

 surface does not extend beyond the limits of the base of the pro- 

 cess, the flat, leafy circumference being tunnelled by straight 

 spacious canals which radiate with great regularity from the root 

 to the expanded border of the fan. In these canals the corpus- 

 cles of the chylaqueous fluid may be observed rolling to and 

 fro, advancing and receding in the same channel. These move- 

 ments are regulated by those of the current in the chamber of 

 the peritoneum. This type of structure prevails in Nereis re- 

 nalis, N. longissima, and in a slightly modified form, in conse- 

 quence of the less flattened shape of the branchiae, in N. viridis. 

 The round or conical variety of branchial process obtains in N. 

 margaritacea, N. Dumerillii, N.fucata, N. pelagica, and A^. bre- 

 vimanus. 



It is difficult to explain why the branchial organs of the Nereids 

 should be entirely destitute of ciliary epithelium. 



The laminated or foliaceous type attains the point of maxi- 

 mum development in the branchial appendages of the genus 

 Phyllodocc. Anteriorly to the discovery of the vital and organic 

 characters of the chylaqueous fluid, the real uses of the rich leafy 

 expansions ornamenting the sides of these attractive Annelids 

 could only have been rudely conjectured. They look more like 

 oars than branchiae, natatory more than respiratory organs. In 

 the absence of correct ideas tending to a knowledge of the nature 

 and capabilities of the fluid contents of the visceral chamber, the 

 real meaning of the radiating channels by which the respiratory 

 laminae are perforated, and therefore of the mechanism of the 

 function of which they are the scene, never could have been 

 rightly apprehended. It was only by mistaking tlie chylaqueous 

 fluid for the true blood that the branchial office of these appen- 

 dages could have been predicated, and this very mistake has 

 been committed by M. Quatrefages. 



The branchiae in Phyllodoce viridis are prominent dorso-lateral 

 appendages ; in this worm the blood-system can be traced only 

 to a few scanty vessels distributed over the roots of these pro- 

 cesses : nor ai'e the canals very spacious and distinct ; they are 

 moi'e like lacunse in a spongy tissue. 



In P. bilineata and P. lamelligera, radiating passages, distinct 

 from each other and communicating only indirectly through in- 

 termediate cells, are readily observed. They carry the visceral 

 fluid, the corpuscles of the latter being seen flowing and ebbing 

 in the same channel. Nothing can, however, more conclusively 

 prove the true branchial character of these laminae than the pre- 

 sence of cilia, the vibrations of which can be observed only at 



