242 LECTURE XI. 



this is the case also with the young of the larger species ; in others 

 they vary from the single vessel to the number of six or eight. 



" 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 return- 

 ing vessels being composed of large-celled tissue, lacunose, into 

 which the peritoneal fluid penetrates by a flux and reflux move- 

 ment. The branchice in L. Ninetta are situated dorsally, and are 

 supplied at their bases with single rows of vibratile cilia. Those of 

 Aglaura fulgida are similarl}'- 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, consti- 

 tuting 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 anatomist encounters a structure strikingly different 

 from anything hitherto described. Whether round or laminated, the 

 true branchifB in this genus are always penetrated by the fluid of the 

 visceral cavity, and the blood-vessels assume a peculiar disposition. 

 When the branchial process is conical in shape, its base is embraced 

 by a reticulated plexus of true blood-vessels, which is situated quite 

 superficially and immediately beneath the epidermis. These vessels 

 are most prominently developed on the dorsalmost process, which, 

 therefore, may be called the branchial ; but they extend more or less 

 over all the cirri. A better characteristic of the branchice, both the 

 conical and the foliaceous, in the Nereids, is that of their being pene- 

 trated by the peritoneal fluid. In those species in which the bran- 

 chial 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 reiialis, 

 the step of the exterior surface does not extend beyond the limits of 

 the base ; the flat portion, however, is tunnelled by straight spacious 

 canals, which radiate with great regularity from the base to the ex- 

 panded 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 peritoneum. This 

 type of structure prevails in Nereis renalis, N. longissima, and in a 

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



