29 
can often be distinguished as a brighter spot in the middle of them. 
Not unfrequently a smaller or a greater number of these cells have 
coalesced. They make the air-sacs very conspicuous in the living 
hyaline animal, in which 
these pigment-cells to- 
gether with those of the 
eyes and a few slightly 
brownish cells in the 
muzzle are the only co- 
loured parts. 
The air-sacs of the 
lake form are present 
in the same number and 
in the same segments as 
those of the pond form; 
but their typical shape 
is a different one. If the 
shape of those of the 
pond form can be descri- 
bed as that of a distor- 
ted kidney, the form of 
the sacs of the lake larva 
might perhaps better be 
compared with that of a 
sausage the one end of 
which is turned down- 
wards and the other cur- HF VE 
ved inwards to form the Fig. 2. Air-sacs of the ike - hente irer 
beginning of a spirale. ne eee Earle as rå fig. 1. Ca. 
It is an exaggeration of 
the extreme curved forms of the sacs of the pond larva described 
above; the inwardly flexed ends are the same as in that larva 
(Fig. 2). Tne posterior sacs are always a little shorter than the 
anterior ones, but the absolute size of both pairs is greater than 
that of the corresponding sacs of the pond larva. The average 
length of the anterior pair is about 0,85—0,9 mm., of the poste- 
rior pair about 0,8—0,85 mm. As it will be seen, the upper limit 
of variation, both as to form and to size, of the sacs of the pond 
