ON THE BRITISH ANNELIDA. 201 



divide the roots of the feet. * From this relation of contact between the peri- 

 toneal fluid and the digestive CEeca, loMch are alioays Jilled hy a dark green 

 chyle, it is impossible to resist the conclusion that the contained fluid is 

 really a reservoir wherein the oxygen of the external respiratory current, 

 already described, becomes accumulated. From the peritoneal fluid the 

 aerating element extends in the direction of the caeca, and imparts to their 

 contents a higher character of organization. These contents, thus prepared 

 by a sojourn in the casca of the stomach, become the direct pabulum for 

 replenishing the true blood which is distributed in vessels over the parietes 

 of these chylous repositories. The sequence of events now indicated will 

 convey to the mind of the physiologist a clear idea of the mechanism of 

 the processes both of respiration and sanguification. It cannot have escaped 

 observation that there prevails a striking resemblance between the general 

 anatomy of Aphrodita aculeata and that of the Asteridse among the Echi- 

 noderms. The point of junction thus established between the Echinoderms 

 and Annelida is as obvious and natural as that which exists between the 

 SipuncididcB and the Nemertinidce. It is thus constantly observed by the 

 philosophical anatomist that in the animal kingdom adjacent classes are 

 linked together into a continuous series at more than one point. 



In the genus Palmyra no external respiratory appendage is provided ; 

 although theyee^ of the Aphrodite are absent, the elytra in the elegant Pal- 

 myra aurifera generate a true branchial current. 



This observation is also true of the family of the Polynoe. These worms 

 are destitute of external branchial processes. The fleshy cirri, by which the 

 true respiratory appendages of Sigalion Boa are represented in the Polynoe, 

 are solid, not hollow and ciliated, and further situated only on every third or 

 fourth foot. The f}.uid of the peritoneal cavity in these worms is voluminous ; 

 it is little corpusculated, like that of ^. aculeata, and moreover the stomach in 

 the genus Polynoe is more or less extended laterally in form of diverticula. 

 The organisation of the familiar Sea Mouse therefore conveys exact ideas 

 with reference to the principles of structure on which nearly all the scale- 

 clad worms are formed. In all ' the scales ' are mechanical, and very skil- 

 fully contrived instruments for generating true branchial currents. In Siga- 

 lion Boa, however (fig. 20, «), which is a worm considerably more elon- 

 gated than the Polynoe, an exception occurs to the principle observed in 

 the other scale-clad worms ; i. e. express external organs of respiration are 

 provided. They exist under the character of hollow, cylindrical and curved 

 appendages (fig. 20, «) emanating by a mammilla under cover of the scales, 

 and projecting a short distance beyond their outer edges. These processes are 

 profusely lined within, but not without, by vibratile cilia. The corpuscles of the 

 peritoneal fluid may be readily brought under the eye, while whirling in the 

 interior. In Polynoe semisquamosa, Williams (fig. 21, a), a flat appendage 

 is added to the base of the foot, presenting radiating canals for the exposure 

 of the peritoneal fluid. It is thus then established by direct demonstration, 

 that the fluid contained in the great visceral cavity is the real and exclusive 

 subject of the process of oxygenation in these scale-armed Annelids. If this 

 fluid consisted only of pure water, that is, if its specific gravity were iden- 

 tical with that of the external element, those conditions would exist which 

 are least favourable to the interchange of oxygen and carbonic acid. It is 

 therefore no departure from cautious reasoning to infer that the oxygen 

 received into the peritoneal fluid exerts upon the elements of the latter the 

 effect of raising them in the scale of organic fluids, and of preparing them 

 for the work of solid nutrition. 



It now remains to consider the mode in which the process of breathing 



