26 
living as it does in considerable abundance in many small lakes 
and pools, it has been made an object of several important stu- 
dies and monographies dealing with its anatomy, metamorphosis and 
physiology (Leydig, Weismann, Pouchet, Krogh etc.). 
Until the year 1898 this larva was only known to live in rather 
shallow lakes and ponds, inhabiting the upper strata of the water, 
living under a pressure of water not exceeding 1 atm., as a rule 
barely reching "/> atm. And as Krogh has shown, this pond-form 
is seriously injured or even killed by a higher pressure than I 
atm. (10 meter below the surface), its air-sacs which are exclu- 
sively of hydrostatic function, being then compressed. — In that 
year however C. Wesenberg-Lund found the larvæ living near 
the bottom at the greatest depths (30—40 m.) of some of the 
Danish lakes, thus enduring a pressure of 3—4 atm., and what 
is still more astonishing, when taken to the surface, after a couple 
of hours they again acquired the perfect equilibrium with the water 
at a depth of only a few ctm.s, showing absolutely no sign of any 
suffering from the highly altered condition of water pressure. 
Later on, this lake form was found also in German lakes (Se- 
ligo) and in lakes of Switzerland (Jak. Schneider, F. Zschokke), 
in some cases living at a depth of 70 m.!) — Seligo indicates 
that the breathing tubes of the pupa differ from those of the pond 
form in being longer and petiolate; this does not hold good of the 
Danish form. 
In the year 1914 C. Wesenberg-Lund compared the larvæ 
of the lake form with those of the pond form and found the for- 
mer differing from the latter in several characters which he sum- 
marizes as follows: 
»1.. As lesser average size. 
2. A greater slenderness in the bodily structure. 
3. A far greater transparency (perfectly hyaline, newer yellow). 
4. A considerably smaller eye. 
5. A less developed tail-fin. 
") Schneiderl.c The writer only indicates that he found the larva in 
a net taken from this depth, but as the animal seems always to live near 
the bottom, I think there can hardly be any doubt that this specimen 
really was taken about 70 m. under the surface. It lived for some days 
in an aquarium. Cfr, pag. 37. 
