ON THE BRITISH ANNELIDA. 197 



them form exclusively the branchial organs : loose and large-celled tissue is 

 superadded to the proper blood-vessels, which are far less in relative size 

 than those in the former variety of branchiae ; into the cells of this tissue the 

 fluid of the visceral cavity insinuates itself, its course being marked by a slow 

 motion. There exists however another point of structural difference between 

 the branchial organs of this group and those of the former ; this difference 

 admits of the following general expression, — that whei'ever the fluid of the 

 peritoneal cavity is admitted into the interior of the branchial organs, the 

 latter are invariably supplied more or less profusely with vibratile cilia. 



In the genus Lycidice the branchia consists of a flat, lanceolate process, 

 more or less developed, surrounded marginally by a blood-vessel, the mid- 

 space between the lines of the advancing and returning vessels being com- 

 posed of large-celled tissue, lacunose, into which the peritoneal fluid pene- 

 trates by a flux and reflux movement. The branchiae in L. Ninetta are situ- 

 ated dorsally, and are supplied at their bases with single rows of vibratile 

 cilia. Those of Aglaura fulgida are similarly constructed, although they 

 differ slightly from those of the former genus in size and figure. In CEnone 

 maculata they occur under a more developed form, constituting flattened, 

 pointed trowel-shaped processes, the plane of which is vertical with reference 

 to that of the body. A blood-vessel, as in the former cases, trends along the 

 borders, immediately beneath the cuticle. The course of these vessels is 

 followed by a row of large and prominent vibratile cilia. 



In the branchial system of the genus Nereis (Cuv.), Lycoris (Savigny), 

 the minute anatomist encounters a structure strikingly different from any- 

 thing hitherto described. Whether round or laminated, the true branchiae 

 in this genus are always penetrated by the fluid of the visceral cavity, and 

 the blood-vessels assume a peculiar disposition. When the branchial pro- 

 cess is conical in shape, its base is embraced by a reticulated plexus of true 

 blood-vessels (fig. 14, a, a, a, a), which is situated quite superficially and 

 immediately beneath the epidermis. These vessels are most prominently 

 developed on the dorsal-most process, which therefore may be called the 

 branchial, but they extend more or less over all the cirri. A better charac- 

 teristic of the branchiae, both the conical and the foliaceous, in the Nereids, 

 is that of their being penetrated by the peritoneal fluid. In those species in 

 which the branchial process is round, the interior of the base is hollow, and 

 filled with the fluid of the visceral chamber. Floating in this fluid may be 

 seen, when viewed transparently,coils of naked blood-vessels; in those in which 

 they are laminated or foliaceous, as in Nereis renalis, the step of the exterior 

 surface does not extend beyond the limits of the base, the flat portion, how- 

 • ever, tunnelled by straight spacious canals (fig. 14, b, b), which radiate with 

 great regularity from the base to the expanded circumference of the process. 

 In these canals the corpuscles of the peritoneal fluid may be seen rolling to 

 and fro, advancing and returning in the same channel. These movements 

 are regulated by those of the great current in the chamber of the perito- 

 neum. This type of structure prevails in Nereis renalis, N. longissima, 

 and in a slightly modified form, in consequence of the less flattened shape of 

 the branchiae, in N. 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 maximum deve- 

 lopment in the branchial appendages of the genus Phyllodoce (fig. 15). 

 It was difficult to assign any other than a respiratory use to the rich, leaf- 

 like projections in these beautiful worms. In the absence of all ideas 



