Alimentary Canal of Arenicola 63 



ALIMENTARY CANAL: BURROWING. 



The alimentary canal presents an almost uniform structure 

 throughout the genus, but the associated glands the oesophageal 

 caeca exhibit certain differences which are of value in diagnosis. 

 The canal consists of (1) an eversible buccal mass and pharynx ; 

 (2) a cylindrical oesophagus, often transversely wrinkled, which 

 pierces the three septa, and, a short distance behind the level of the 

 last of these, bears two or more oesophageal glands; (3) the 

 "stomach," which is covered with yellow cells, gradually merges, 

 about the level of the eleventh or twelve segment, into (4) the 

 intestine, which extends to the posterior end of the worm and 

 opens at the anus to the exterior. 



During life the first part of the alimentary canal, the " proboscis," 

 is being constantly everted and withdrawn carrying sand or mud, with 

 the contained vegetable and animal organisms, into the oesophagus. 

 During eversion the buccal mass is first extruded, armed with 

 numerous rounded, conical or triangular (Fig. 45, p. 106) vascular 

 papillae ; afterwards is everted the pharynx, covered with small 

 rounded processes, which give it a mammillated appearance. In 

 large examples of A. marina the pointed tips of the buccal papillae are 

 provided with shining, black or brown, chitinoid caps. There is no 

 other armature associated with the alimentary canal of Arenicola; 

 jaws are entirely absent. 



The oesophageal glands, which form a valuable diagnostic feature, 

 open into the posterior part of the oesophagus. In A. pusilla and 

 assimilis there are several of these glandular caeca on each side of 

 the oesophagus (PJ. XIII, Figs. 44, 45); in all the other species only 

 a single pair is present (Pis. IV, IX, VIII, Fig. 17). In A. marina, 

 cristata, glacialis and loveni the glands are generally conical in shape, 

 but in some cases, owing to dilatation of their anterior portion, they are 

 clavate. They are usually relatively smaller in A. cristata than in 

 other species. In A. ecaudata and branchialis the oesophageal glands 

 are clavate or flask-shaped, being almost always dilated anteriorly. 

 In the two species, A. pusilla and assimilis, in which several glands 

 are present, the most anterior caecum on each side is considerably 

 longer than the rest and has usually thinner walls. This anterior 

 gland is generally finger-shaped and may be nearly an inch in length ; 

 the others are more or less pear-shaped or ovoid and are much 

 shorter, some of them being only about a millimetre in length. 



All the species of Arenicola burrow in sand or gravel. Burrowing 



