I] 



PERIODICITY IN CONVOLUTA 



25 



ing, the larvae, after circling actively within the 

 capsule, burst the walls thereof and escape. Thus 

 they have, during the remainder of the neap-tidal 

 period, some days of comparative tranquillity and 

 uniformity of conditions. Not for some days yet will 

 they be exposed to the full to the chances and changes 

 which must beset their adult lives. They are born 

 as submarine animals, and in their earliest days are 

 spared to some extent the buffetings which shall 

 be theirs when, with the advent of the spring tides, 

 they are, now, clinging to fragile weed dashed against 



Fig. 8. Periodicity of egg-laying and hatching of C. paradoxa. The 

 shaded band shows the position of the Paradoxa zone with 

 respect to low water-marks of spring and neap tides. The 

 undulating line, joining up the low water-marks of successive 

 day-tides, is obtained by marking off," along the verticals indi- 

 cating successive days, from a zero line above, the amount of 

 vertical descent (in decimeters) of each day's tide. On those 

 days when the undulating line falls below the shaded band, the 

 Paradoxa zone is uncovered during low water ; on those days 

 when the low water-line lies above the shaded baud, the zone is 

 continuously submerged. The dots represent egg-capsules, the 

 crosses signify larvae hatched ; the positions of dots and crosses 

 give the dates on which the capsules were laid and larvaB 

 emerged. 



