ON THE BRITISH ANNELIDA. 225 



necessary therefore to resort to the general principles of the science of Com- 

 parative Anatomy, while elucidating the uses of these divisions of the di- 

 gestive system in the Annelida which dissection discloses. The external 

 organs (tentacles) appended to the head, have already been shown to be 

 used by some species in the prehension of food. In others they subserve 

 only the purposes of touch and general protection to the head. 



The proboscis, though used also for the seizure of the I'ood, bears not to 

 the tentacles the remotest anatomical analogy. It is now proposed to enter 

 at length upon the consideration of the conformation, anatomical structure 

 and physiological meaning of the numerous varieties traceable in the organs 

 devoted to the processes of digestion and assimilation in this class of 

 animals. 



The genera SerpulcB, Sabellce and Amphitritce, are distinguished by the 

 fact, that they subsist on fluid food ; if not absolutely so, on those minute 

 particles of organic matter which perchance may float on the surrounding 

 water. This water is directed in a perpetual stream towards the mouth, by 

 means of the vibratile cilia with which in part for this purpose the branchiae 

 are provided. The current thus dashed against the mouth is swallowed, and 

 the suspended particles of food arrested in the digestive organs, while the 

 water, which was used only as a mechanical vehicle for the conveyance of 

 the food into the interior of the body, is rapidly carried along the intestinal 

 canal, and ejected through the extreme outlet of the body, situated at the 

 bottom of the tube, in streams sustained in rapid motion by the ceaseless 

 vibration of definitely disposed cilia. This water current, traversing in the 

 manner indicated the whole interior length of the body, constitutes ' a fact ' 

 of great consequence in the ceconomy of these tubicolous Annelids. It is 

 through its agency that the feculent refuse is projected from the bottom to the 

 upper orifice of the tube, and that the habitation of the worm is maintained 

 in a state of never-varying cleanliness and purity. The rectum or tail extre- 

 mity of the intestine in these genera is lined internally, and to some distance 

 inwards from the anus, with large and vigorous cilia, which at this situation 

 reinforce the current descending along the intestine and drive it outwards 

 with great force. 



In the SerpulcB, the pharynx and oesophagus extend from the oral orifice 

 to the termination of what may be called the thoracic segment of the body ; 

 whereat is situated a crop-like dilatation of the canal. The pharynx and 

 oesophagus are beset on their internal surface with numerous minute follicles, 

 which contribute a secretion having some digestive property. The walls of 

 this segment are delicate and membranous. At the ' crop,' however, the 

 parietes present increased density, having become more strongly muscular. 

 The uses of this portion of the canal are manifestly those of crushing minute 

 fragments of stones, sand-particles, &c. which may be swallowed ; that this 

 description is swallowed by the Serpulce, may be proved conclusively by the 

 observation of the sandy and earthy contents of the intestine posterior to the 

 crop. Like the corresponding structure in graminivorous birds, it is in these 

 worms a crushing and grinding engine. It is curious, however, to remark, 

 that it does not exist in some few species of Sabellce, so nearly akin in organ- 

 ization to the Serpulce; it is present in Amphitrita alveolata (Plate XI. 

 fig. 50 a), absent in Sabella chlorema, absent also in S. vesiculosa. The pre- 

 sence of this dilatation of the canal is ordinarily indicated externally by a 

 bulge of the body. From the annulus, coinciding in situation with the crop, 

 the true intestine begins (fig. 50 h) ; this portion of the alimentary tract in 

 nearly all Annelids, is characterized by a bright yellow-coloured, streaked 

 by a rich network of strikingly red blood-vessels, composing a surface of 



1851. Q 



