198 REPORT — 1851. 



tending to a knowledge of the nature and capabilities of the fluid con- 

 tents of the visceral chamber, the real meaning of the radiating channels 

 (fig. 15, a, a, a) 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 the peritoneal fluid 

 for blood that the branchial office of these appendages could have been pre- 

 dicated, and this very mistake has been committed by M. Quatrefages. The 

 branchiae in Phyllodoce viridis are prominent dorsal appendages : in this 

 worm the blood-system can be traced only by a few scanty vessels distributed 

 over the roots of these processes ; nor are the canals very spacious and di- 

 stinct ; they are more like lacunar in a spongy tissue. In P. bilineata and 

 P. lamelligera, the radiating passages, distinct from each other, and communi- 

 cating only indirectly through cells, are extremely obvious under the micro- 

 scope (fig. 15, «, a, a). They carry the fluid of the peritoneal cavity, the 

 corpuscles of which may be seen flowing and ebbing in the same channel. 

 Nothing can, however, more conclusively prove the true branchial character 

 of these laminae than the presence of cilia, the vibrations of which can be 

 observed only at the edges of the respiratory laminae. These are best seen 

 in P. lamelligera. This is a striking point of distinction between the Phyl- 

 lodoce and the Nereids, in which vibratile cilia on the branchiae have no 

 existence. The peritoneal fluid then may now be affirmed as that, in the 

 ceconomy of the Phyllodoce, which is the subject exclusively of the respira- 

 tory function, the true blood receiving its supply of oxygen from this fluid, 

 afterwards to convey it to the solid structures of the body. 



In the genus Glycera the blood-proper is entirely excluded from the organs 

 of respiration. This office devolves exclusively on the chyl-aqueo.us fluid, 

 which in nearly all the species of this genus is profusely supplied with red 

 corpuscles. The gills consist of hollow cylindrical appendages (fig. 16, a), 

 emanating from the base of each dorsal foot at its superior aspect, filled in 

 the interior with the fluid of the visceral cavity ; but, what is remarkable in 

 the structure of these organs and quite peculiar to this genus, is that the 

 interior parietes of the cylindrical hollow of the branchiae is lined with 

 vibratile cilia ; these motive organules cause the corpuscles of the fluid by 

 which the branchiae are penetrated, to move with great rapidity in a definite 

 direction, viz. peripherally on one side and centrally along the other, each 

 corpuscle whirling on its own own axis as it proceeds. Ciliary vibration 

 cannot be detected on the outside of the branchial appendage. 



It is a feature of structure more strikingly illustrated in Glycera than in 

 any other Annelid, that whenever the peritoneal fluid is the subject of the re- 

 spiratory function, it is brought into the branchial organ in much greater re- 

 lative proportion than the blood-proper when it is the subject of this process ; 

 and the branchiae are always constructed in adaptation to this difference. 



In the SyllidcB the branchial organs (fig. 17) are penetrated only by the 

 peritoneal fluid, but it can be detected in motion only in the bases of the feet, 

 and these parts only are furnished with vibratile cilia, which are large and 

 active. The long filiform and, in some species, moniliform appendages which 

 are described commonly as the branchiae of these worms, have no central 

 hollow (fig. 17, a) ; they are filled with large-celled tissues through which 

 the fluid parts of the contents of the visceral cavity slowly penetrate. But in 

 the spacious chambers occupying the bases of the feet (fig. 17, b), a whirl- 

 pool of the peritoneal fluid may be readily observed. The structure now de- 

 scribed is very perfectly typified in S. prolifera; the moniliform variety is 

 best seen in S. armillaris and S. maculosa, A similar confirmation prevails 

 in the genera loida and Psamathe of Dr. Johnston. In the Syllidan family^ 



