ON THE BRITISH ANNELIDA. 237 



surfaces of which they crawl in search of food. The oesophageal apparatus, 

 with its appended proboscis, is always powerfully muscular. Some anato- 

 mists have assigned the name of the " gizzard " to the true oesophagus in 

 Aphrodita aculeata. Whatever outward resemblance this part may pre- 

 sent to a gizzard, it possesses none of the structural characters of this latter 

 organ. Its parietes are powerfully muscular and dense, because they are the 

 engine by which the proboscis is thrust out and drawn in. The extended 

 proboscis of ^. aculeata is a savage-looking instrument; in size it is propor- 

 tionate to that of the animal, and is edentulous. The oral orifice of this 

 proboscis is encircled by a short and thick-set fringe of compound peni- 

 cillate filaments, divided into sets by a fissure on each side; each filament 

 has a short stalk, with a tuft of numerous forked papillae on its summit ; ex- 

 terior to the orifice of the proboscis there are four fleshy tubercles placed at 

 the angles. As the external surface of the proboscis and that of the oesopha- 

 gus are devoid of all glandular structure, the iiiternal lining membrane of both 

 these portions of the digestive tube is glandularly organized, that is, the 

 membrane is villose and highly vascular. It is important to remember this 

 fact, for it denotes an anatomical feature which belongs to all the Aphrodi- 

 tacece, embraced in the four leading genera Aphrodita, Polynoe, Pholoe, and 

 Sigalion. Plate X. fig. 62 illustrates the outline anatomy of the alimentary 

 system of Polynoe squamata. It exhibits a striking approach in plan to 

 that of Aphrodita aculeata (c, c, digestive caeca). From its position in 

 the alimentary tube, it is manifest that this villose oesophageal membrane 

 furnishes a fluid which is concerned in the process of digestion ; it does not 

 extend in any species beyond the limits of the oesophagus. The true stomach 

 is quite dissimilarly organized. The proper occasion has now arrived for 

 explaining the real physiological meaning of the complicated csecal append- 

 ages, so familiar to the comparative anatomist, by which the interior capacity 

 of the digestive cavity is multiplied in the AphroditacecB. 



These appendages, in their mechanical arrangement, realized two import- 

 ant objects : — 



1st. They effect the purpose of lodging and detaining a considerable 

 quantity of a dark-greenish chymous fluid. 



2nd. They are so disposed with reference to the large exterior current of 

 water which rushes under the elytra or scales, as to bring their chymous 

 contents as closely as possible into contact with this aerating medium. 



They perform, therefore, two supreme functions, namely, that of respi- 

 ration and digestion. Perhaps this latter process, in this particular in- 

 stance, should be designated as that of sanguification, since the fluid con- 

 tained in these caeca is evidently the blood in its first stage of preparation ; 

 it is' the fluid which, when absorbed into the circulating system, becomes 

 the true blood. It is to the AphroditacecB what the chylo-aqueous con- 

 tents of the peritoneal cavity is to all the other Annelids. This cavity in the 

 Aphrodites, like that of the Echinoderms, is occupied by a fluid, which in 

 appearance and composition closely approaches sea-water. It can scarcely 

 be doubted that this cavity, thus tilled, becomes in these eccentric worms a 

 reservoir wherein oxygen accumulates, and that from this store, in part at least, 

 the fluid contained in the digestive caeca draws its supply of the aerating 

 element. In the AphroditacecB, the blood-proper is colourless, and the blood- 

 system of vessels is very inferiorly developed. The median tube of the 

 stomach, which is straight and unsegmented, and from either side of which 

 the casca proceed, is always found to be filled with a semi-solid faeculent 

 matter, which is quite unlike that contained in the caeca. These two por- 

 tions of the contents of the digestive system are kept apart by a sphincteric 

 structure encircling the openings leading from the straight stomach into the 



