SABELLAMA ALVBOLATA. 



27 



the tuft being from the front slightly upward and backward. In structure these bristles 

 differ from the ventral series in being comparatively stout, perfectly smooth, and with 

 the tips apparently abraded from use. 



Arnold Watson has found in some examples " a centrally situated ciliated cirrus at 

 the outer edge of the membrane between the opercular lobes, as in 8. spinulosa, whilst in 

 others it is absent." He thinks, from observations on very young specimens, that this 

 cirrus is an outgrowth from the upper lip, just in front of the posterior part, and 

 connected to it by a ridge which widens as it approaches the lip. Moreover, on each 

 side of this ridge are the eye-spots, which in young specimens are visible from the 

 dorsum, and apparently rest on a pair of ganglia. They disappear or are hidden by 

 pigment in older forms. As the animal grows the ridge lengthens, and simultaneously 

 the peristomial membrane, which is continuous with it, increases, and apparently in 

 some ensfulfs the cirrus. 



Fig. 138.— Oral region of a small example (f in.) of Sabellaria alveolata showing the building organ bo., with its 

 glandular apertures ; m., muscular bands. From a sketch by Arnold Watson. 



In a young form kept by Arnold Watson under observation, and which had eighteen 

 external opercular palea3 on each side and ten or twelve in the middle row, a distinct, 

 unpaired, short ciliated cirrus, about twice as long as broad, occurred between the lobes 

 terminating the ridge continued from the prostomium. 



In the youngest stage, that is, with two long buccal tentacles and three others on each 

 side, two black eye-spots occur on the ventral side just in front of the bases of the buccal 

 tentacles, as well as two similar spots on the dorsal or inner side of the operculum in 

 rear of the origin of the buccal tentacles, and they form a square with the ventral. In 

 older stages the dorsal eye-specks disappear, but a series of specks is found on each side 

 of the median line of the operculum (Arnold Watson). The same author observed a 

 comma-shaped structure (ganglion ?) on each side of the central ridge at the end 

 nearest the mouth. 



The oral tentacles form a series of converging rows on each side to the number of 

 about twelve, but the number is variable. Each basal lamella is concave in front, and 



