264 ANNELIDA. 



it is impossible to resist the conclusion that the contained fluid is really 

 a reservoir wherein the oxygen of the external respiratory current 

 becomes accumulated. From the peritoneal fluid the aerating element 

 extends in the direction of the ca3ca, and imparts to their contents a 

 higher degree of organization. These contents, thus prepared by a 

 sojourn in the caaca 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, by an 

 arrangement strikingly analogous with what we have already seen in 

 the Asterida3 amongst Echinoderms . 



(678.) In order to arrive at a correct knowledge of the reproductive 

 system of this Annelid, specimens of both sexes should be examined in the 

 spring, and again in the autumn. A good example of a female Aphro- 

 dite being obtained, the dissection should be thus proceeded with : 

 Pin the animal down to the trough with its back upwards. Open it by 

 a longitudinal incision extending from the tail to the head. The in- 

 cision should cut through the scales, felt, and integuments, in order to 

 lay open the spacious perigastric chamber. The integuments should 

 now be carefully stretched, and pinned down to the sides, so as to expose 

 the interior. Let the dissection be then gently floated in salt water, 

 and the parts will present the appearance here described. A network of 

 minute tubes or threads will be seen to twine round and embrace the 

 diverticula of the alimentary canal. 



(679.) The entire alimentary system must next be taken away, and 

 with it necessarily a considerable portion of the reproductive network. 

 A view will thus be obtained of the attached ends or roots of the 

 branched segmental organs. These roots will be found to equal the ali- 

 mentary ca?ca in number, and therefore that of the feet which are situated 

 posteriorly to the proboscidiform oesophagus. They appear under the 

 character of pyriform tubuli, commencing or ending in a single external 

 orifice. Internally they are lined with ciliated epithelium, the cilia 

 being large, dense, and acting with great force and vigour. The current 

 raised by these cilia sets up on one side and down on the other. The 

 ciliated epithelium ceases at the point where the primary branches 

 divide. All the rest of the organ is unciliated, and filled with the re- 

 productive products. This portion is elaborately branched, the branches 

 twining round the diverticula of the stomach. No microscopic object 

 can be more beautiful than a portion of this tubular network. The 

 individual tubes are bridled on one side and glandular on the other. 



A similar structure is exhibited by the male tubes*. 

 (680.) TUBICOLA. The two preceding orders of Annelidans are 

 erratic ; but in the third we find creatures inhabiting a fixed and per- 

 * Dr. Williams, Phil. Trans. 1858, p. 134. 



