ON LAKViE OF LINGULA AND PELAGODISCUS (DISCINISCA). 49 



A larva with 10 pairs of cirri was taken in the Red Sea at locality (i). Its shell- 

 valves are 76 mm. long, and "81 mm. broad, and the internal organs are similar to 

 those of the larva with 8 pairs of cirri described above, except that the coelomoducts 

 (or nephridia ; see below, p. 52) are now formed, and the liver-lobes and intestine 

 more clearly differentiated, but there is still no anus. 



(ii) Larvse with 11 pairs of cirri. — This stage is represented by two specimens. 

 The one studied alive was taken in the Red Sea (locality (i)). The shell-valves of 

 this larva (fig. 5) are "82 mm. long, and '91 mm. broad, and the hinge-lines and the 

 postero-lateral margins of the shell have acquired a brownish-yellow colour, which 

 persists, and, indeed, usually increases in amount in older specimens. The eleventh 

 pair of cirri has been recently formed. The anterior occlusor muscles have increased 

 considerably in size, as compared with those of the previous specimens, and the 

 posterior occlusor* is well formed. The two anterior dorsal liver-lobes are just 

 beginning to grow out from the mid-gut, the intestine is now complete, and the 

 anus is visible, opening into the mantle cavity about the middle of the length of 

 the body-wall on the right side. 



The first indication of the peduncle is seen as a hemispherical elevation, about 

 15/u. high, on the ventral mantle-fold, near its posterior margin, behind and slightly 

 to the right of the posterior occlusor. The peduncle of Lingula is evidently 

 therefore not equivalent morphologically to that of Terebratulina , which is formed 

 at a much earlier stage of development from the entire posterior region (the " third 

 segment " ) of the larva. 



On looking through a series of plankton-samples, obtained from a sea-water 

 bath-tap on board ship by Dr Nelson Annandale in 1901 in the Red Sea and 

 Indian Ocean, I found, in the catch taken in the Strait of Bab-el-Mandeb on 

 March 25, a larva similar in structure to that just described, but larger, its shell- 

 valves being '92 mm. in length and "97 mm. in breadth. 



(iii) Larvw with 12 pairs of cirri. — This stage is represented by four specimens 

 from the Red Sea (locality (i)). The dimensions of their shell- valves are : — 





Length. 



Breadth. 



a 



10 mm. 



10 mm. 



b . 



. 1-06 „ 



1-07 „ 



c 



• 1-1 „ 



1-05 „ 



d . 



• 11 „ 



1-07 „ 



It will be noted that, whereas in the examples previously described the shell- valves 

 are broader than long, in the four now under consideration the length of the shell 

 is equal to or rather greater than the breadth (fig. 3). The shell-valves have 

 developed, in the neighbourhood of the hinge-line, a greenish colour which 

 gradually fades away towards the lateral regions. 



* Although the other muscles of the animal are well shown in the later stages, details of these muscles are not 

 given, as I have nothing to add to Dr Yatsu's account of them. 



