ON THE BRITISH ANNELIDA. 199 



which excels all others in grace and beauty, the proper blood-system is almost 

 indetectable, in consequence of the colourlessness of the contents. It may be 

 stated with confidence that blood-vessels do not enter into the structure of 

 the branchial processes. The respiration therefore devolves exclusively on 

 the chyl-aqueous fluid. 



Amongst the family Ariciadce, first defined by Audouin and Milne-Ed- 

 wards, several other varieties in the configuration of the breathing organs 

 occur. In the genera Leucodore, Nerine and Aricia, the branchial appen- 

 dages affect a dorsal situation. In every species they are traversed from 

 base to apex by a single blood-vessel returning upon itself (fig. 18, a). This 

 vessel, however, is supported by a lobule of spongy tissue (fig. 18, b), into the 

 cells of which the fluid of the visceral chamber penetrates. The office of re- 

 spiration in this family is therefore discharged in part by the blood and in 

 part by the chyl-aqueous fluid. In every species of this family the branchiae 

 are supplied by vibratile cilia having a distinct disposition in each. Lincodore 

 ciliatus, on the dorsal aspect, and over the posterior two-thirds of the body, 

 is covered on either side with a row of flattened conical branchial processes, 

 blood-red in colour and richly ciliated. They are largest anteriorly and small- 

 est near the tail. The cilia are disposed in a spiral line from the attached to 

 the extreme end. Viewed with a high magnifying power, and transparently, 

 a camerated axis, composed of exquisitely fine hyaline cartilage, may be dis- 

 covered, fulfilling on the branchiae of this elegant little boring Annelid the 

 office of mechanical support, as a similar structure was formerly shown to 

 do in those of the Sabellcs. 



In the genus Spio or Nerine the respiratory organs occur under forms of 

 the highest beauty (Plate V. fig. 18). They constitute flat, membranous, 

 penknife-shaped appendages, curving gracefully over the back with the curve 

 of the " ring " of the body by which they are supported, and crossing over the 

 dorsal median line and alternating with the corresponding process of the other 

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

 axis of the body ; they lie therefore one over the other in an imbricate 

 manner. 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 posteriorly. The blood-vessels, the aff"erent and efferent (a, fig. 18), 

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

 part of each is composed of a membranous lobular addition to the inferior 

 and vascular portion. Into the cells of this lobule the chyl-aqueous fluid 

 slowly finds its way, and participates obviously in the office of respiration. 

 In N. coniocephala it is remarkable that the cilia should be limited in their 

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

 fact is less manifest in N. vulgaris in consequence of the smallness of the 

 membranous lobule. In Aricia Cuvieri the branchial appendages are more 

 conical in figure, more vertical in position, and developed only at the poste- 

 rior four-fifths of the body. They are covered with large vibratile cilia, which 

 likewise extend over that segment of the dorsum which separates the bases 

 of the branchiae. Like those of the preceding genera, they are supplied with 

 spongy tissue (fig. 18, b) for the exposure of the peritoneal fluid*. It may 

 have been remarked that in all the members of the preceding family the real 

 branchial organ has consisted of an evolved or exaggerated development of 

 the superior element of the dorsal foot. In the genus Nephthys, which 

 comes now under review, it is the inferior element of the dorsal foot which 

 becomes the subject of this evolution. Nephthys Hombergii oi our coasts 



* I have described a species of Aricia in which these branchial organs are entirely sup- 

 pressed. 



