308 The Entomostraca of Mid-Lothtan. [Sess. 
capturing it. A large number of them crawl at the bottom 
of ponds, and cannot swim at all. They have all a steady, 
even method of progression, quite unlike the jerky movements 
of a copepod. 
The cladocerans we found particularly interesting. They 
are very varied, and often present most elaborate and beauti- 
ful developments, especially in the antennas and the post- 
abdomen. They swim with a slow, jerky motion, rising 
slightly in the water at the commencement of each stroke, 
and sinking ‘at its close. Many of them maintain an up- 
right position, with the head uppermost; and this, of course, 
makes it much more difficult for them to progress quickly. 
This is very noticeable in the daphnids; the lynceids, which 
swim with the head depressed and on the same level as the 
body, move much more rapidly. 
The body of a cladoceran is enclosed in one large shell, 
which is open on the ventral and posterior margins. The 
head is protected by a shelly covering, firmly united to the 
rest of the shell; the line of junction can often be traced, and 
in some species, after a specimen has been dead any length of 
time, the two portions of shell become entirely separated. 
The “legs” are used for breathing; and the organs used in 
swimming are the antennas and postabdomen. 
Habits. 
As a rule, the freshwater entomostraca are fond of sunlight, 
and we have generally made our best catches on bright days. 
It has happened to us more than once, that after finding 
Ravelston Quarry literally swarming with Diaptomus gracilis 
and Bosmina longirostris on a bright day, we have returned a 
few days later in dull cold weather and been unable to find 
half a dozen specimens of either. The quarry is deep, and in 
dull weather the creatures presumably retreat to the deep 
water. Many species, including the two just mentioned, 
‘ prefer clear water; others, as certain of the daphnids, and 
many of the ostracods, seem to thrive best in muddy water. 
The copepods and ostracods may be found all the year 
round; but many of the cladocerans appear to die off in 
the winter. 
