ON LARV^l OF LINOULA AND PELAGODISCUS (DISCINISCA). 61 



joint situated a short distance proximal to the middle of its length, and the flattened 

 and tapering distal portion exhibits faint indications of several joints, and bears 

 along one margin minute spicules, which, however, do not extend to the tip of the 

 chseta (fig. 13).. When at rest these chsetse usually point antero-medially, but in 

 preserved specimens they may be found directed antero-laterally owing to the 

 contraction of the muscles attached to their bases. The third chseta is usually 

 gently curved, about '09 mm. long, and is directed postero-laterally. It bears on 

 its anterior edge minute, regularly arranged, pointed processes, which begin a little 

 proximal to the middle of the length of the chseta., but do not extend to its tip 

 (fig. 14). The fourth chseta is the largest, being "S-^ mm. long, and much thicker 

 than the others. It issues between the shell-valves postero-laterally and then 

 usually curves towards the middle line, as shown on the right side of fig. 11, but 

 it may be turned so as to point outwards, as on the left of fig. 11. Its proximal 

 portion — about one-fifth — which is almost uniform in diameter, merges into a dilated 

 region where the chseta issues between the two shell-valves ; distal to this the 

 chseta tapers gradually to its tip. The greater part of the tapering portion is beset, 

 except on its medial side, with numerous pointed teeth or thorns, the tips of which 

 are directed postero-laterally (fig. 15). The thorns do not extend quite to the tip 

 of the chseta ; a terminal portion about 15/a long is without them and is very fragile. 

 The four pairs of chsetse described above arise from the ventral mantle-fold. The 

 fifth chseta on each side, which arises from the dorsal mantle-fold, is about 

 •18-'2 mm. long. It is slender and strongly curved so that its tip points postero- 

 medially, and it bears, on its distal half, minute spinules similar to, but smaller 

 than, those of the fourth chseta. Muscles are attached to the bases of each of these 

 five chsetse. 



There is also on each side a series of about thirty chsetse, all of similar form, 

 regularly arranged in close order in the thickened zone of the dorsal mantle-fold, 

 and extending from the region of the fourth principal chseta forwards almost to the 

 middle line. These chsetse are almost uniform in diameter along the greater part 

 of their length, but taper rapidly at their tips. They exhibit nodes at intervals, and 

 are flexible ; they are usually directed postero-laterally, and their free ends are bent 

 under the edge of the ventral valve. Only the two most anterior and the most 

 posterior of these chsetse (Ch. S.) are shown on the right of fig. 11. 



On each side are two or three thicker chsetse situated in the ventral mantle- 

 fold, about midway between the second and third principal chsetse, and directed 

 almost laterally (fig. 11, right side). Each of these bears longitudinal striations 

 and is subdivided by nodes so that it resembles a bamboo (fig. 16). . 



The peduncle, as seen in the living or stained specimen (fig. 11), is an oval mass, 



the transverse and antero-posterior diameters of which are, on an average, about "13 



and "04 mm. respectively. In vertical section (fig. 17) the peduncle appears as an 



elevation, triangular in section, of the inner wall of the ventral mantle-fold close to 



TRANS. ROY. SOC. EDIN., VOL. LI, PART I (NO. 3). 10 



