GLYCERA ALBA. 489 



passes from side to side between the edges of the dorsal and ventral longitudinal muscles 

 under the proboscis. This seems to take the place of the oblique muscles in this region. 



The first foot has a single group of two spines, three small lobes, the superior being 

 the shorter, and it bears both dorsal and ventral bristles which extend beyond the lobes. 

 The dorsal basal papilla appears on the first foot and it is conspicuous on the second. 

 The ventral lobe is the shortest in this foot, the two upper being about equal in length. 

 The spines diverge distally, each going to its bunch of bristles (dorsal and ventral), so 

 that in the succeeding feet each spine is connected only with its own group of bristles. 

 The foot thus early shows two spinigerous lobes, two long lobes superiorly extending 

 considerably beyond these, a shorter lanceolate lobe interiorly, and the basal papilla 

 dor sally. 



In its passage backward the dorsal lobe or process, which first appears as a papilla 

 in the anterior region, assumes a more or less erect position, until in the typical foot it 

 arches upward and. outward. Moreover, its structure differs from that of the other lobes 

 which are hypodermic, for it shows a close series of spiral or crossed fibres, evidently 

 muscular, and which are probably connected with the respiratory functions of the parts. 



Beneath the foregoing are two flattened lanceolate lamellse — between which the dorsal 

 bristles emerge — supported at the base by their spine. These bristles are translucent 

 and faintly yellowish (Plate LXXXV, fig. 5), with a curvature at the tip and a very 

 slight enlargement, after which the bristle tapers to a fine point. 



The ventral bristles again emerge between a short blunt posterior lobe and a 

 lanceolate flap in front. The bristles (Plate LXXXVI, fig. 5 a) form two groups which 

 slope gently from the spine. The ventral lobe is bluntly lanceolate, and reaches only as 

 far as the tip of the truncate lobe above. The dorsal papilla is short, with a constriction 

 at the base. The ventral spine is considerably longer than the dorsal. 



At the tenth foot (Plate LXXVI, fig. 3) the dorsal lobe is long and with an 

 enlargement at the base dorsally. The other anterior lobe is narrower and somewhat 

 longer, whilst the ventral is shorter and somewhat conical. The upper posterior lobe is 

 papilliform and considerably shorter than the two anterior lobes. The lower posterior lobe 

 is also somewhat pointed or papilliform in this region, but afterwards becomes less acute. 

 The basal papilla (the dorsal cirrus of Ehlers) forms a rounded knob above the base of 

 the foot dorsally. 



A considerable change occurs at the twentieth foot, for the slight elevation at the 

 base of the dorsal papilla of the foot has now assumed the form of a short branchia 

 (which is first visible about the twenty-fourth or twenty-fifth foot) extending more than 

 half the length of the upper papilla. The other lobes are similar to those in front except 

 that the lower posterior lobe is now shorter and more truncated. The changes noted are 

 still more evident at the thirtieth, the fiftieth, and the seventieth foot, the last (Plate 

 LXXVI, fig. 3 a) presenting a typical condition. The dorsal papilla or cirrus is now on 

 the dorsal base of the foot; the branchia extends considerably beyond the tip of the 

 papillse. The surface of the organ is corrugated, and a series of crossed fibres (Plate 

 LXXVI, fig. 3 b) which indicate the contractile condition of the parts is evident. The 

 various lobes are typical and stand out prominently. 



Towards the tip of the tail the branchia diminishes to a small process at the base of 



119 



