Tiesjjiration in Invertebrate AiiiiiuiL. 401 



ciliated blood-vessels no longer in 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 flux and reflux motion. There exists, 

 however, another point of structural diflference between the 

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

 difierence admits of the following general expression — that 

 wherever the chylaqueous fluid is admitted into the interior of 

 the branchial organs, the latter are invariably supplied more or 

 less profusely with vibratile cilia. 



In the genus Lysidice the branchia consists of a flat, lanceo- 

 late process, more or less developed, surrounded marginally by a 

 blood-vessel, the mid-space between the lines of the advancing and 

 returning vessels being composed of large-celled lacunose tissue, 

 into which the chylaqueous fluid penetrates by an advancing 

 and i-eceding movement. The branchiae in L. Ninetta are situated 

 dorsally,andare supplied at their bases with singlerows 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 (Enone 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 case, 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 (Cuvier), Lycoris 

 (Savigny), the minute anatomist encounters a structure strikingly 

 dissimilar from anything hitherto described in the Annelids. 

 Whether round, or laminated, or foliaceous, the true branchiae in 

 this genus are always penetrated by the chylaqueous fluid, and 

 the blood-vessels assume a peculiar disposition. ^Mien the 

 branchial process is conical in shape, its base is embraced by a 

 reticulated plexus of true blood-vessels, which are situated quite 

 superficially and immediately beneath the epidermis. These 

 vessels are most prominently developed on the dorsal-most pro- 

 cess, which therefore may be called the branchial, but they extend 

 more or less over all the cirri. A better characteristic of the 

 branchiae in the Nereids is that of being penetrated by the fluid 

 of the visceral chamber. In those species in which the branchial 

 process is conical, the interior of the base only is hollow and filled 

 with chylaqueous fluid. Floating in this fluid may be readily 



* See plates in the author's Report on the Biitish Annelida, Trans, of 

 Brit. Assoc, for 1851. 



Ann.^Mag.N.Hist.ScT.2. Vol.xn. 28 



