54 



DR J. H. ASHWORTH 



Comparison of the Larvae with those previously recorded. 



The larvae described in the foregoing account fall into a single series, and 

 evidently all belong to one species. On comparing them with those described by 

 Dr Yatsu, there can, I think, be no doubt they belong to the same species — Lingula 

 anatina Bruguiere, — which is, so far as I am aware, the only species recorded from 

 the Indian Ocean. In structure and colours my larvae agree closely with Dr Yatsu's, 

 and special points of agreement are : the hinge-line is practically the same length 

 in both series of larvae, and the chaetae evidently arise in the same manner, for it is 

 clear from Dr Yatsu's figure (e.g. fig. 86) that in his larvae, as in mine, the two 

 chaetae first formed in the postero-lateral region remain for a considerable time 

 distinguishable from those formed later. 



Captain Sewell concluded that his larvae also were those of L. anatina. 



There are certain differences between my larvae and those of Dr Yatsu and 

 Captain Sewell which may be now considered. 



(a) Size. — The following table summarises the average dimensions of the shell- 

 valves of the larvae of different stages : — 



No. of Pairs of 



Yatsu. 



Sewell. 



ASHWORTH. 



Cirri present. 



Length. Breadth. 



Length. Breadth. 



Length. Breadth. 



5 



•31 x -39 mm. 







6 



•44 x -41 „ 







7-8 



•66 x "61 „ 



(December Series.) 



"52 x *62 mm. 



9 





•65 x - 66 mm. 





10 





•82 x -77 „ 



•76 x -81 „ 



11 





•89 x -80 „ 



•87 x -94 „ 



12 







1-06 x 1-05 „ 



13 





. . . 



1-26 x 1-15 „ 







(February.) 





14 





I'll x I'll mm. 



1-48 x 1-22 „ 



15 



•8 x - 64 mm. 





/ 1-52 x 1-22 „ 

 1 1-6 x 1-37 „ 









Dr Yatsu's larva? with 7 or 8 pairs of cirri were rather larger than mine, 

 but his later stages were much smaller — compare them when they have 15 pairs 

 of cirri. This disparity is no doubt due to the fact that Dr Yatsu's larvae, from 

 those with 8 or 9 pairs of cirri onwards, were kept in aquaria, and the growth of 

 the shell was retarded, for it may be noted that the shell-valves of his specimens 

 were not only smaller but much thinner (see p. 51 and footnote). Captain Sewell's 

 larva with 14 pairs of cirri is considerably smaller than mine, but as he suggests that 

 the conditions at the time of the year when it was taken were unfavourable, the 

 difference in size may be due to this cause. 



(/>) Change in the Shape of the Shell. — The table serves to show that the change 

 in the shape of the shell-valves — that is, when they become for the first time longer 



