Gatty Marine Laboratory, St. Andrews. 9 



canals, which can be traced into nerve-cells. An intricate 

 scries of fibres in transverse section occurs in the middle 

 line between the nerve-cords and surrounds a small granular 

 area above and another below. In each segment (probably 

 at the junction) a very complex series of fibres — chiefly 

 transverse and oblique — commingle over the nerve-area, 

 whilst in the intermediate regions the ventral vessel and the 

 muscular fibres and mesentery attached to the lower edge of 

 the alimentary canal are more distinct. The alimentary 

 canal itself is normal in section, and it has large blood-sinuses 

 and vessels on its wall, besides the dorsal trunk (in its region). 

 The thoracic glands occur in front, and the segmental organ 

 lies to the exterior of the ventral longitudinal muscle. 



Toward the posterior end, whilst little change takes place 

 in the hypoderm and the ventral subhypodermic belt, or in 

 the circular muscular coat, the dorsal longitudinal muscles 

 are considerably extended laterally, whereas the ventral 

 longitudinal muscles are diminished in transverse diameter 

 and have the bristles close to their outer edge. The nerve- 

 cords occupy the same position at the inner edge of the muscles 

 and next the circular coat, the neural canal having about 

 the same proportional size as in front. The complex crossing 

 of fibres above the area occurs at intervals as in front. The 

 gut in this region is filled with dark sandy mud. 



Branchial Apparatus. — One of the most interesting featur s 

 in the structure of the Sabellids, such as Bispira volutacornis, 

 Montagu, is the chordoid skeleton which supports the bran- 

 chial apparatus, and which commences behind the brain as 

 a small lateral area (PI. III. fig. 15, ch.), which soon develops 

 into an arc ou each side (PL I. fig. 1, ch.). About the region 

 of the brain the lateral arcs fuse in the mid-dorsal line 

 (PI. I. fig. 2, ch.) and thus form a continuous curved belt from 

 side to side, not, however, of uniform breadth in a given 

 section, but with indentations, as at the large coelomic area 

 dorsad of the brain or at the enlargements laterally. This 

 chordoid tissue is finely reticulated in the adult, more 

 distinctly cellular in the young, the connecting walls staining 

 slightly, and nuclei are very evident, especially in young 

 examples. It is bounded externally by the firm investment 

 or "perichondrium/"' thebasement-tissue and muscular layers, 

 hypodermic and articular, whilst internally it is bounded by 

 the same homogeneous border of "perichondrium " to which 

 muscles are attached. This " perichondrial " boundary 

 (PI. II. fig. 10, pr.) is not a separate layer, but processes from 

 its inner edge all round pass as bridles to the reticulations 

 and cells composing the interior, so that the two are modi- 

 fications of the same ti-suc, the whole organically connected 



