ON LARVAE OF LINGULA AND PELAGODISCUS (DISCINISCA). 53 



Growth in length of the shell-valves is evidently taking place at this stage much 

 more rapidly than growth in width, as the later growth-lines show (fig. 4). The 

 shell has now become elliptical, the longer axis being antero-posterior. In specimens 

 a and b there are 14 pairs of cirri; in c and d the number of pairs is apparently 

 15, but as the cirri are closely packed together it is difficult to be certain of their 

 number. 



Additional chaetae have been developed in the postero-lateral region of the 

 mantle, where there is now a group of about six or eight long chaetae on each side, 

 among which the two original ones may generally be distinguished by their slightly 

 greater length and thickness (fig. 6). Other shorter chaetae have been added at 

 intervals round the lateral and anterior margins of the mantle. 



The posterior occlusor is now a round or oval cylinder of muscle about 80-100/x 

 in diameter. The peduncle is from - 4 to "8 mm. in length,* and is bent into one or 

 two loops, its terminal part lying dorsal to the proximal part, and its tip pointing 

 to the left (fig. 6). The terminal portion of the peduncle is dilated, and the 

 epithelium covering this region is much higher than elsewhere, but it has not yet 

 proceeded to form the secretion which later envelops it. The peduncle, which is 

 colourless, is in each specimen enclosed within the shell-valves, no part of it being 

 extended. 



The median sensory tentacle does not exhibit the least sign of reduction ; it is 

 still large, about 160/x long, and is evidently fully functional. 



These specimens are the largest and latest free-swimming stages recorded for any 

 species of Lingula (s. lat.). Probably these larvae would soon have settled down, 

 the peduncle of each would have been extended beyond the valves, and the high 

 epithelium of its terminal portion would have produced secretion by means of which 

 the larva would have fixed itself to the substratum. 



A note of the colours exhibited by the living larvae may be given here. The 

 shell-valves, even of the largest larvae, are transparent, there being no calcareous 

 matter present. As already mentioned, the posterior and neighbouring lateral 

 margins of the valves are yellowish brown, and in these regions the zone just within 

 the margin is a bright green colour. The mantle-margin is usually brownish, 

 especially posteriorly. The basal half of the tentacle, particularly on the dorsal 

 side, and the distal portions of the "cirri, are yellowish brown, and there is a patch 

 or spot of deeper tone at the tip of most of the older cirri, and a similar spot 

 about the middle of the dorsal surface of the tentacle. There is yellow pigment 

 on each side of the mouth at its postero-lateral margins. The " liver "-lobes are 

 lemon-yellow, and near their periphery a small amount of brown pigment is present. 



* The largest specimen seems to be abnormal in this respect, for the peduncle is comparatively short — about 

 •2 mm. in length,— but it is not possible to state its length exactly, owing to its being much foreshortened, as seen 

 in the preserved specimen. 



TRANS. ROY. SOC. EDIN., VOL. LI, PART I (NO. 3). 9 



