ANNIE MATTHEWS. 
-5 0 
On the fourth day the cells near the centre of the solid pre- 
planula begin to disintegrate, and so the larva again becomes 
hollow and passes on into a free-swimming planula stage with 
a definitely marked anterior pole. This planula developes 
cilia, is at first roughly spherical and of sinuous outline (Text- 
fig. 4), but lengthens somewhat in a few hours into a highly 
contractile oval planula still of wavy outline. By the fifth 
day it is a smooth oval planula swimming rather slowly at 
various levels in the water, usually in a horizontal plane 
(Text-fig. 5). Very soon the anterior end broadens and a 
pear-shaped planula results, which rotates continuously on its 
long axis while progressing in the water (Text-fig. 6). The 
reddish -yellow colour of the ovum is still present, but gradu- 
ally becomes paler as the yolk is absorbed, and the planula 
increases in length. The larva continually changes its shape, 
so that measurements of the ever varying length and breadth 
are rendered difficult. While swimming it exhibits charac- 
teristic planarian-like contractile movements, which are 
represented in Text-figs. 6 and 7, and any irritation causes 
strong contraction and lateral wrinkling (Text-fig. 8). 1 By 
the seventh day the planula is very long and slender, measuring 
T3 mm. long and 03 mm. wide, but is not very often fully 
extended. The anterior and aboral pole is deeper in colour 
than the narrower posterior and oral pole where more yolk 
has been absorbed (PI. 3, fig. 1). The surface at this time 
is abundantly supplied with nematocysts and mucous cells 
(PL 3, fig. 2), the latter being especially numerous at the 
anterior pole. At first the planulae swim at varying levels in 
the bowls, but towards the third free-swimming day they 
become more sluggish, and most of them keep in a vertical 
position with the thin aboral end hanging downwards (c.f. 
<le Lacaze Duthiers ( 10 ), PI. xiii, fig. 6). Many then sink to 
the bottom of the dishes in this position (possibly this is an 
1 All the young stages are very sensitive to heat, and the microscope 
lamp has to be used with caution while examining them or they quickly 
die. 
