404 Dr. T. Williams on the Mechanism of Aquatic 



the blood-proper system is almost indetectible, in consequence 

 of the colourlessness of the contents. The blood does not parti- 

 cipate in the function of breathing : it is an office performed ex- 

 clusively by the chylaqueous fluid. 



Amongst the family Ariciada several other varieties in the 

 configuration of the breathing organs occur. In the genera 

 Nerine (fig. 8) and Aricia the branchial appendages affect a dorso- 

 lateral situation : they are traversed in every species from base to 

 apex by a single blood-vessel returning upon itself (i) . This vessel 

 is supported by a lobule of spongy tissue (/), into the cells of which 

 the chylaqueous fluid insinuates itself. In every species of this 

 family the branchiae are supplied by vibratile cilia, exhibiting in 

 each a distinct disposition. In ultimate structure, those of Am- 

 phitrite alveolata and Leucodore ciliatus, remote specifically from 

 the Syllidans, display the same construction. 



In the genus Spio, abundantly common on our shores, the 

 respiratory organs occur under forms of the highest beauty. They 

 constitute flat membranous penknife-shaped appendages curving 

 gracefully over the back and crossing over the median line, 

 alternating imbricatively with the corresponding processes of the 

 other side. The plane of each process is vertical in relation to 

 the long axis of the body (fig. 8). They are less flat and close in 

 N. vulgaris than in N. coniocephala ; they are largest in size 

 towards the middle of the body, smallest anteriorly and poste- 

 riorly. The blood-vessels, the aff"erent and efi'erent, run close 

 to and parallel with the inferior border of the process ; the upper 

 part of each is composed of a inciubranous lobular {(J,f) addition to 

 the inferior and vascular portion. Into the cells {y) of this lobule 

 the chylaqueous fluid slowly finds its way, and participates ob- 

 viously in the office of respiration. In N. coniocephala it is re- 

 markable that the cilia should be limited in their distribution 

 to the margin along which the true blood-vessel runs. This 

 fact is manifest in N. vulgaris in consequence of the smalluess 

 of the membranous lobule. In Aricia Cuvieri the branchial 

 appendages are more conical in figure, more vertical in position, 

 and developed only at the posterior four-fifths of the body : they 

 are covered with large vibratile cilia. Like those of the former, 

 they are supplied with flat lobules of spongy tissue. In all the 

 members of the preceding family the real branchial organ consists 

 of an evolved or exaggerated development of the superior element 

 of the dorsal foot. 



In the genus Nephthys (fig. 7, A) which comes now under re- 

 view, it is the inferior element of the dorsal foot which becomes 

 the subject of this evolution. It is a curved conical process (a), 

 attached to the inferior aspect of the root of the upper foot. It 

 is hollow and filled with the chylaqueous fluid, the corpuscles of 



