AXNULATA. 



243 



the branchiae, in JV. viridis. The round variety of branchial processes 

 obtains in N. margaritacea, N. Dumerillii, N. fucata, N. pelagica, 

 and N. brevimanus. It is a fact difficult to explain that the branchial 

 organs in the Nereids should be destitute in every species of vibratile 

 cilia. 



" The laminated or foliaceous type attains the point of its maxi- 

 mum development, in the branchial appendages of the genus Phyllo- 

 doce. Rich leaf-like projections, it was difficult to assign any other 

 than a respiratory use to these parts in these beautiful worms. 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 distinct, they are more like lacunae in a 

 spongy tissue. In P. hilineata and P. lamelligera, the radiating 

 passages, distinct from each other and communicating only indirectly 

 through cells, are extremely obvious under the microscope. They 

 carry the fluid of the peritoneal cavity, the corpuscles of which may 

 be seen flowing and ebbing in the same channeh Nothing can, how- 

 ever, more conclusively prove the true branchial character of these 

 laminae than the presence of cilia, the vibrations of which can be ob- 

 served only at the edges of the respiratory laminae. These are best 

 seen in P. lameUigera. This is a striking point of distinction be- 

 tween the Phyllodoce and the Nereids, in which vibratile cilia on the 

 branchiae have no existence. The peritoneal fluid, then, may be 

 affirmed as that, in the economy of the Phyllodoce, which is the 

 subject exclusively of the respiratory function, the true blood receiv- 

 ing 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 

 chyle-aqueous fluid, which in nearly all the species of this genus is 

 profusely supplied with red corpuscles. The gills consist of hollow 

 cylindrical appendages, emanating from the base of each dorsal foot 

 at its superior aspect, filled in the interior with the fluid of the vis- 

 ceral 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 bran- 

 chise are penetrated, to move with great rapidity in a definite direc- 

 tion, viz. peripheradly on one side and centradly along the other, 

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

 vibration cannot be detected on the outside of the branchial appen- 



dage. 



K 2 



