ON LARVtE OF LINGULA AND PELAGODISCUS (DISCINISCA). 57 



The three free-swimming larvae figured by Brooks have shell-valves with the 

 following dimensions : — 



Shell-Valves. 

 No. of Pairs of Cirri. Length. Breadth. 



5-6 '23 mm. -27 mm. 



7-8 "29 „ -29 „ 



9-10 -4 „ -37 „ 



These laiwae, and especially the two last, are much smaller than those of Lingula 

 anatina with the same number of cirri. The distance between the teeth at the ends 

 of the hinge-line on the ventral valve in Brooks's larvae is i5-"16 mm., as compared 

 with about '3 mm. in L. anatina ; and the antero-posterior length of the protegulum 

 of the dorsal valve is about "065 mm. in Brooks's larvae (figs. 1 and 3), and '12 mm. 

 in L. anatina. 



Ckaetae are formed in the larvae of Glottidia when about 9 pairs of cirri are 

 present, and are shown (Brooks's fig. 3) all of the same size and sparsely scattered 

 round the margin; there is apparently not a larger first-formed pair on each side 

 as in L. anatina (see p. 50). It will be noticed that the chaetae appear about the 

 same time as the change in the shape of the shell-valves takes place, as was also the 

 case in L. anatina. 



The posterior occlusor and the peduncle are formed when 7 pairs of cirri are 

 present, and by the time there are 10 pairs of cirri the peduncle has become long 

 and looped, and Brooks expressed the opinion that the larvae became sedentary 

 soon after this stage, but the change was not actually observed, as it was found that 

 " the larvae could not be made to thrive in confinement." 



The youngest sedentary example of Glottidia found has 16 pairs of cirri. The 

 shell-valves of this specimen are about 2 mm. long and 1 mm. broad, and the 

 peduncle — now fully protruded — about 5-6 mm. long. In the dorsal and in the 

 ventral mantle-lobe there are four pigment-spots near the anterior margin ; there 

 are no pigment-spots in my larvae of Lingula l - 52 and 1'6 mm. long respectively, 

 nor do they appear to have been present in Dr Yatsu's specimens. 



The most noticeable differential characters of Brooks' specimens of Glottidia 

 are : in the free-swimming stages, the small size of the shell-valves, the short hinge- 

 line and consequently the small protegulum ; in the sedentary specimen, the narrow 

 shell-valves * and the presence of pigment-spots near the anterior mantle-margin. 



Brooks's larvae of Glottidia were obtained in Chesapeake Bay, and as young 

 stages were found in the middle of July, while only older -larvae were taken in the 

 middle of August, the breeding season was probably short, as was also found by 

 Dr Yatsu for Lingula at Misaki, which is in nearly the same latitude as 

 Chesapeake Bay. 



* In this connection it may be remarked that full-grown examples of Glottidia audelarti do not attain so laige a 

 size as those of Lingula anatina, and the shell-valves of the former are narrower in proportion to their length. 



