192 REPORT — 1851. 



extremely exaggerated at the head ; this fact is exemplified in the Syllidce. 

 The tentacular cirri of the Nereidce are instances of the same development. 

 Both the fleshy and branchial appendages in the dorsibranchiate Annelids 

 are more or less suppressed in the ventral feet. 



The further study of these complex and compound organs, the appendages, 

 will be more advantageously prosecuted under the threefold division of, — 1st, 

 the branchial ; 2ndly, the tactile and locomotive ; and 3rd, the setae. 



Branchial Processes. — In nearly all the species of the genus Serptila, the 

 true branchiae are grouped around the cephalic extremity, in two divisions, 

 one on either side of the mouth ; the feet in these tubicolous worms are com- 

 posed exclusively of the setae. The branchial processes are remarkably com- 

 plex in their minute structure (Plate IV. fig. 11, A). Projecting in a comb- 

 like form from the head, and tinted variously and beautifully in different 

 species, they ai-e admirably adapted for the exposure of the blood to the influ- 

 ence of the surrounding medium. Each process is supported by a camerated 

 frame or basis (fig. 1 1 , A, a), large and distinct in the back of the comb, from 

 which are sent off, on one side only, a double row of secondary processes, 

 corresponding to the teeth of the comb. This supporting framework is com- 

 posed of an extremely delicate and flexible cartilage, the chambers of which 

 are filled by a limpid fluid, which is in communication with that of the peri- 

 toneal cavity ; an afferent and efferent blood-vessel, in parallelism, accompany 

 this frame-structure. In the secondary processes (fig. 11, A") the two ves- 

 sels (b, b') are brought towards the inferior aspect, to which the vibratile cilia 

 are in some species limited. 



The cilia are large and vigorous, and cause the current, resulting from 

 their vibration, to set strongly in the direction of the mouth. The branchiae 

 therefore in the genus Serpula are rendered at once, by virtue of their pecu- 

 liar structure and situation, subservient to the two grand oftices of respira- 

 tion and prehension. By these sedentary Annelids, and that necessarily from 

 the nature of, and the mode in which they obtain their food, a large quantity 

 of water is swallowed ; this circumstance suggests an explanation of the fact 

 that in the Serpula and Sabellce the interior of intestine throughout the 

 rectal or posterior third of its extent, is lined by active vibratile epithelium. 

 By the ceaseless agency of these cilia, a projecting force is imparted to the 

 fluid emerging at the inferior orifice, which reacting against the bottom of 

 the tube, assumes the direction of an upward tending current, and maintains 

 the tube in the best sanitary condition, and the animal always, within and 

 without, in contact with a constantly renewed stream of fresh water. When 

 the animal is about to retire into its cell, the branchiae are furled into a 

 compact ball, which is drawn under cover of the strong membranous hood 

 situated at the base of the branchial tuft, and the whole compressed and pro- 

 tected by the retracted operculum. More minutely watched, the process of 

 furling the branchiae discovers other refinements of mechanism. Each 

 separate secondary process is first rolled upon itself into a minute concentric 

 coil ; this movement begins at the extreme end of each process, and rapidly 

 creeps towards the base, at which moment the axis or vertical shaft rolls 

 concentrically upon itself, and every trace of the gill disappears, so exqui- 

 sitely perfect is the packing. By this movement of folding, both the blood- 

 movement and the vibration are arrested. The process of unfurling is the 

 reverse of that described (fig. 11, A). 



The preceding description applies almost in every minute particular to the 

 ultimate structure of the branchial appendages of the Sabellce, in which 

 geaus these organs affect a corresponding cephalic situation, — in Sabella d 



