ON LARVAE OF LINGULA AND PELAGODISCUS (DISCINISCA). 55 



than broad — took place at different stages in each of the series of larvse under con- 

 sideration. In Dr Yatsu's this change occurred when 7 or 8 pairs of cirri were 

 present, in Captain Sewell's specimens collected in December the change took 

 place when 9 pairs of cirri were fully formed, and in my larvse when 12 pairs of 

 cirri were present. Captain Sewell's single specimen taken in February had a 

 shell as long as broad ; he attributes the delay in change of the shape of the shell* 

 to the less favourable time of the year when development was taking place. 



(c) Chwtte. — Chaetse appeared in Dr Yatsu's larvae at an earlier stage than in 

 mine, but in both series the time of appearance of the chsetse coincided with the 

 change in shape of the shell- valves. 



{d) Peduncle. — In Dr Yatsu's larvae the peduncle appeared when 6 pairs of cirri 

 were present, and was protruded by the time 10 pairs of cirri had been formed, at 

 which stage his larvse, kept in aquaria, became fixed. Perhaps the artificial con- 

 ditions brought about precocious fixation. In Captain Sewell's larvae the peduncle 

 began to develop in the stage presenting 9 pairs of cirri, but there was no sign of 

 its protrusion in his specimens with 11 pairs of cirri. In my larvae the first trace 

 of peduncle did not appear until 11 pairs of cirri were present, and the peduncle 

 was entirely internal in the largest larvae, which had 15 pairs of cirri and were 

 still free-swimming. 



(e) Tentacle. — In my largest larvse, with 14 or 15 pairs of cirri, the median 

 sensory tentacle was not in the least degree reduced, but was fully functional, 

 whereas in Dr Yatsu's examples with 15 pairs of cirri — the specimens being then 

 attached to the bottom of the aquaria— the tentacle was either absent or reduced to 

 a small papilla. Dr Yatsu regards the tentacle as a larval organ, and therefore 

 considers its disappearance in his fixed specimens as a natural consequence. There 

 is, however, the possibility that the conditions of captivity had determined the 

 reduction and eventual loss of this sense-organ in his larvse. A similar tentacle is 

 present in Glottidia ; and in Brooks's figure (fig. 7) of a young specimen, with 16 

 pairs of cirri, soon after it had become sedentary, the tentacle is shown as long as 

 the neighbouring cirri, and Brooks states definitely (p. 73) that it is persistent. 



Most of the differences between the various series of larvae discussed above, 

 particularly in reference to the size of the shell-valves, the size and protrusion of the 

 peduncle, and perhaps the persistence or loss of the tentacle, are doubtless dependent 

 almost entirely on the environmental conditions. Variations in one or more of these 

 characters may be expected in larvae of the same species obtained in widely separated 

 areas and under diverse conditions. There are, however, certain features which 

 appear to be more constant for the species, at any rate they present a fairly close 

 agreement in Dr Yatsu's larvae and mine f — namely, the length of the hinge-line and 



* The peduncle of this specimen was also in a backward state, being " still only a small rudiment." 

 t In Captain Sewell's larva; there was apparently a rather greater range of variation in the length of the hinge, 

 but its average length in his twelve larvae was '3 mm., practically the same as in Dr Yatsu's and mine. 



