REPORT ON GEPHALODISCUS DODECALOPHUS. 15 



fig. 2, pi ; PI. VII. fig. 5, pi), that covering the ventral surface being denser and more 

 finely granular, and with a more definite margin, which is probably richly ciliated. This 

 denser and somewhat regularly streaked hypoderm (which also stains more readily) shows 

 several prominent frills or rugas where it joins the oral region, and it just turns the 

 outer edge of the lamella all round and then ceases. The dorsal layer of hypoderm on 

 the other hand is more lax, and is thrown into a series of frills or crenations in the 

 preparations, the streaks in it being more conspicuous than the granules. It resembles, 

 indeed, the somewhat lax hypoderm observed on the pedicle, and like the latter contains 

 numerous pigment-corpuscles which do not readily stain. So far as the structure can 

 form a guide, the ventral surhice would seem to be more important functionally than the 

 dorsal. 



The oral region therefore has a different environment from that in Rliabdopleura, 

 though the plan of structure follows parallel lines. Thus in the excellent figures of 

 Lankester,^ a well-marked plate situated behind the mouth, and running into the buccal 

 disk in front of it, is apparently the homologue of this lamella. When the disk is 

 folded backwards {op. cit, fig. 2) the two surfaces come more or less into contact, and 

 would thus send currents more surely into the mouth. Lankester does not allude to 

 this region, which lies just in front of his thoracic division in Rhahdopleura. In the 

 flattened surface of the post-oral lamella the buccal shield is closely applied in the 

 preparations, though in life they can of course be separated at will, thus permitting the 

 currents caused by the cilia of the opposed surfaces to reach the oral aperture. As its 

 posterior face has perhaps only to perform the function of separating the currents 

 connected with the mouth from those of the gill-slits, the difi"erences in structure are thus 

 explained. 



The post-oral lamella may have some relation to the Molluscan foot, and also to 

 the post-oral ring of cUia in Polygordius, especially as a ciliated groove in the latter 

 runs between it and the mouth. Harmer's view that it is homologous with the oper- 

 culum of Balanoglossus, as described in Bateson's valuable and suggestive papers ^ on this 

 form, appears to be well founded. 



Digestive. System. 



Mouth. — The margin of the oral lamella leads on each side (PI. III. fig. 3 ; PI. VI. 

 fig. 2, m) into the mouth, and in some ventral views it passes straight inwards to the 

 sides of the latter, and forms a transverse margin anteriorly. The edges of the mouth 

 are slightly raised or frilled laterally and posteriorly, the latter often being spout-shaped. 



1 Op cit., pi. xxxviii. figs. 1, 2. 



2 Quart. Journ. Micr. Sci., April 1884 ; also Studies from the Jlorph. Lab. Univ. Camb., vol. ii. part i., and vol. iu. 

 part i. The author has overlooked some remarks pre\'iously published iu this country on Balanoglossus. Vide 

 Nemerteans, Bay. Soc, 1873, p. 144. 



