ON THE BRITISH ANNELIDA. 203 



rapid, and composition highly organized ; in every sense it is organically and 

 chemically qualified for the discharge of this function. 



By Audouin and Milne-Edwards, in this place in the Annelidan series 

 is placed the anomalous genus Clymene. No attempt has been made by these 

 distinguished authors to unravel the anatomy of these eccentric worms. On 

 the head and body no vestige of external appendage (except the hooks) is 

 discoverable. At the tail, however, irregularly scalloped, membranous pro- 

 cesses may be observed, which are in every essential respect to these worms, 

 what the cephalic processes in them are to the SipunculidcB ; that is, they are 

 hollow membranous projections of the peritoneal cavity, admirably adapted 

 to expose the contents of this cavity to the influence of the surrounding 

 medium. A blood-vessel or two may be traced at the roots of these pro- 

 cesses, being only enough to prove that the system of the blood-proper in 

 these Annelids is very insufficiently developed to enact the great function of 

 respiration. The branchial processes of the Clymenidce are not provided with 

 cilia ; they afford the only illustration in the class Annelida of branchial 

 organs specialized around the outlet of the alimentary system. 



It has now been shown that the branchial organs in the Annelida arrange 

 themselves under two leading divisions, between which a clearly legible line 

 of demarcation exists. Under one of these divisions, the blood-vessels bearing 

 branchiae occur ; under the other, those organized for the exposure of the 

 chyl-aqueous fluid. It was formerly demonstrated in detail that this fluid, 

 in larger or smaller volume, and in proportions varying in different species 

 relatively to those of the true-blood system, occurs in nearly every known 

 Annelid. When the contents of the latter are well exposed to the agency of 

 oxygen, no provision in general exists for the exposure of the former fluid ; and 

 conversely, when the appendages designed for the office of breathing are con- 

 structed with especial reference to the outspreading of the former to the 

 aerating medium, blood-vessels are seldom found to enter into their structure. 

 It is probable therefore that in the ceconomy of the Annelid these two fluids 

 are co-ordinate elements; they are convertible proximate principles; they 

 exhibit equal physiological capacities ; both are capable of discharging the 

 function of respiration, and both are capable of supplying the solids of the 

 body with the materials of increase. 



Locomotive and tactile appendages. — The cirri and setae of the feet are 

 included under this head. The former admit of subdivision into two varieties 

 of which one may be classed as the natatory and the other as the tactile. 

 The setae, constituting in many species appendages to the body, of the most 

 brilliantly ornamental character, are always important mechanical means of 

 locomotion. Sensation and locomotion are thus in the Annelida provided 

 for by means of organs of elaborate construction. So numerously and inter- 

 estingly diversified in number, proportions, and form, are these several parts 

 of the feet, that a detailed description of them becomes here necessary in 

 order that what is most characteristic in the anatomy of species may be fully- 

 expounded. That element of the foot which is dedicated to the function of 

 breathing has already been made the subject of minute inquiry. Under the 

 present head therefore the branchice will receive no further notice. 



The branchicB and operculum which plume so richly the head in the ge- 

 nus Serpula, are endowed with extreme sensibility. This provision super- 

 sedes the necessity lor fleshy tactile appendages to the body, which is enclosed 

 by the calcareous tube in which the worm lives : in structure and curvature 

 these tubes differ in different species. The interior is smooth, not so smooth 

 however as to be slippery, nor so hard as to render difficult or impossible the 

 fixing of the hooks and bristles of the feet, by which the animal is enabled to 



