50 DR J. H. ASHWORTH 



Each of the larvae has 12 pairs of cirri, but in the smallest specimen (a) the 

 most anterior cirrus on each side is a newly formed conical papilla about 40/x long. 

 That the tips of the cirri can be protruded beyond the edges of the shell-valves was 

 observed in life, and is further evidenced by two of the preserved specimens, in each 

 of which one of the cirri has been " trapped " in the tightly closed valves, so that its 

 tip is still projecting. 



The posterior margin of the mantle, which even in the earliest larva examined 

 was slightly pigmented, now exhibits clearly a yellowish-brown pigment, which 

 extends forwards along the lateral mantle-margin. 



Chsetse have been formed on each side in the postero-lateral region of the mantle. 

 Two slender chgetas first appear close together. Along the middle and distal regions 

 of each chteta are nodes, which in the middle portion of the chseta are at fairly 

 regular intervals of about 8ju, but near the tip they are closer together (cf. fig. 7) ; 

 in the basal part of the chseta they are less clearly marked. By the time these 

 chsetre have increased in length to about 200-250/a, a few others of similar structure 

 but shorter (50-70/a long) have been formed at intervals along the lateral and 

 anterior margins of the mantle. About this time also gland-cells appear in the 

 postero-lateral portions of the mantle in a zone a little removed from its margin ; 

 these gland-cells become much more abundant in the next stage (fig. 6), and finally 

 extend all round the mantle, forming a glandular ridge. The area of the mantle 

 median to this ridge is very thin and transparent. It may be noted here that the 

 mantle is highly contractile, and, in a few of the preserved specimens, is withdrawn 

 a considerable distance from the margins of the shell-valves. The radially arranged 

 muscle-fibrils, by which the mantle is fixed to the shell, are well seen in most 

 specimens. 



The peduncle is bluntly conical in form and about 60-70//, long. Its wall consists 

 of a thick outer epithelium and an inner layer which is continuous with the ccelomic 

 epithelium (fig. 9). There is a central cavity which opens into the general 

 body-cavity (fig. 6). 



(iv) Larvae with 13 pairs of cirri — Six specimens at this stage of development 

 were found along with the preceding examples in the Red Sea (locality (i)). The 

 measurements of their shell-valves are — 





Length. 



Bread tli. 



a 



1*17 mm. 



1 ' 1 5 mm. 



b . 



. 1-22 „ 



113 ,, 



c 



• 1-28 ,, 



1-14 „ 



d . 



. 1-29 „ 



1-15 „ 



e 



. 1-29 „ 



1-17 „ 



/ . . 1-3 „ 1-15 „ 



The maximum dorso- ventral diameter of each of the last four specimens is "2 mm. 



The peduncle shows a marked increase in size during this stage; in the first 

 two specimens {a, b) it is about 80/x long, but in the others it is 200-250/x long, 



