16 THE VOYAGE OF H.M.S. CHALLENGER. 



Anteriorly it is devoid of any well-defined boundary other than the attachment of 

 the pillar of the great buccal shield, and leads directly upwards into the alimentary canal 

 (PL III. fig. 1, m). Moreover, as Mr. Harmer first noticed in his sections, a sohd 

 diverticulum proceeds upwards dorsally into the stalk of the buccal shield, and this may 

 fairly be held to be the homologue of the notochord of Balanoglossus. In transverse 

 section it is nearly circular and presents a somewhat regular arrangement of its cells, so 

 that a concentric appearance is frequently present. In longitudinal section, on the other 

 hand, the process, which is small, has a slightly bent clavate outline, a dotted axis 

 indicating the lumen continued from the alimentary canal. The mucous membrane of 

 the buccal chamber and gullet are thrown into many prominent rugse, from the dense 

 glandular nature of the tissue. The latter is especially thick on the ventral side of the 

 mouth (the region lying in front of the post-oral lamella), and a strong layer of muscular 

 fibres passes to the basement-tissue of this region, which must thus possess considerable 

 mobility. The surface of the mucous membrane is apparently richly cUiated, the 

 cylindrical epithelium of which it is composed being so closely arranged as to give a 

 finely striated character to the tissue. In some preparations a thin film occurs on the 

 surface of this glandular layer, but this is evidently due to mucus, and not to the 

 separation of a superficial or cuticular coat. 



Pharyngeal Region. — Beneath the post-oral lamella and immediately behind the 

 pores of the second region are a pair of gill-slits, which were first clearly recognised as such 

 by Mr. Harmer, who has kindly interested himself in the structure of this form, and 

 whose very thin sections enabled him to unravel certain points wliich would otherwise 

 have been obscure. The folds leading to these in transverse section are shown in 

 PI. VI. fig. 2, gs. Immediately behind the collar-pores the ordinary hypodermic coatino- 

 of the body becomes continuous with the translucent wall of the slits, which seems to be 

 a modified continuation of the pharyngeal mucous membrane. The granules are finer, 

 and the whole tissue is more translucent. It also does not stain so well as either the 

 collar-pores or the pharyngeal lining proper. In connection with this structure it is 

 interesting to note that Bateson^ mentions that the gill-slits in Balanoglossus arise as 

 dorso-lateral evagiuations. As soon as the posterior boundary of the mouth is completed, 

 and this is easily recognised in the preparations by the appearance of the pigment-ceUs in 

 the dorsal layer of the hypoderm of the post-oral lamella as it now stretches right across the 

 ventral surface, the spacious pharynx presents a thickly folded wall of the same kind of 

 minutely glandular tissue. The projection of some of these thick folds of glandular 

 tissue under the post-oral lamella, sometimes causes peculiar appearances in transverse 

 section, as if special diverticula existed. Bounding this thick glandular wall externally 

 is a firm basement-layer, probably of a highly elastic nature, and it is to this coat that 

 the muscular fibres formerly alluded to are attached. 



^ Ox'. cit., p. 14. 



