SABELLAKIA ALVEOLATA. 29 



The dorsal surface is marked by close transverse striae and an ovoid area about 

 the sixth and seventh branchiae. Ventrally the groove at the same level has a 

 similar depressed ovoid area, and the caudal tube lies in the groove posteriorly, whilst 

 anteriorly the groove commences behind the first (or thoracic) region. Arnold Watson 

 counted about thirty rows of long cilia on one of the longest branchial filaments. 

 They arise from grooves like the dorsal cilia, and apparently shorter cilia occur in 

 the intervals. 



Behind the fragmentary segment just described is the first region of the body 

 proper (thoracic auctorum), consisting of three segments, each with a dorsal branchia and 

 a lateral setigerous lamella, which increases in length and breadth from the first to the 

 third. The first is least, and has slightly smaller oar-shaped bristles (Plate CXXIII, 

 fig. 3 c), seven in number, the pointed translucent tips of which are only a little fringed, 

 and thus differ from those of Sabellaria spinulosa with their deep fissures. The second 

 has a longer lamella than the first and a few bristles more, and the third exceeds both in 

 these respects, and the bristles often show asymmetrical tips. The shafts of all these 

 oar-shaped bristles are remarkably long, penetrating deeply into the tissues, and they 

 also appear to be flat. Between each, as a rule, is a slender finely spinous form 

 (Plate CXIII, fig. 3 d) 9 only the tip of which projects from the surface, but it has a long, 

 straight shaft almost as large as that of the oar-shaped kind. The bristles are stained 

 in life by methylene-blue (Arnold Watson). The posterior margin of the first and second 

 lamellae is coarsely crenate, but that of the third is smooth, whilst a ridge from each 

 lamella passes downward to the ventral bristles. 



The ventral bristles, as in the previous species, conform to the type of the dorsal, 

 being more slender forms with oar-shaped tips. 



The posterior region of the body has about thirty-four bristled segments, and 

 tapers gently from the anterior to the posterior end, but the latter is of considerable 

 breadth and is gently rounded to join the caudal tube. The dorsal surface is more or less 

 flattened and marked by a rather broad median band with transverse grooves, which are 

 not opposite those between the lateral lamellae — indeed, they alternate with these. The 

 ovoid anterior area is opposite the first, second, and part of the third segments of 

 the region, whilst posteriorly the median band rises into an elevated rounded ridge, 

 continuous with the caudal tube. The branchiae form conspicuous dorsal processes, the 

 first seven or eight being noteworthy for their great length. Their structure corresponds 

 with that observed in 8. spinulosa. The ventral surface has a deep median groove, 

 commencing with the first segment of the region and continuing to the last, for the 

 reception of the caudal tube as it curves to this surface. Two transverse riclges cross 

 the groove in front of the anterior area, the second being slightly bent forward, 

 and the next (third) curved more boldly backward to suit the ovoid boss. As with 

 the grooves on the dorsum, these transverse ridges are opposite the middle of each 

 segment. 



This region (second body-region) has no dorsal bristles. A ridge passes from each 

 branchia laterally and ventrally, almost to the edge of the ventral groove. The upper 

 third of the ridge is flat, then the torus for the uncini projects and ends ventrally in a 

 papilla; lastly, the ridge is continued ventrally, and ends in a process for the ventral 



