48 MR. R. GURNEY ON THE LIFE HISTORY OF THE CLADOCERA. 
Ceriodaphnia pulchella, it is covered with little elevations of 
the chitin. 
The Sididae and Polyphemidae lay their resting eggs 
protected only by their thick shell. The Lynceidae have 
ephippia, but generally the part of the shell from which 
they are formed is only slightly modified. It becomes darker 
in colour and somewhat thickened, but that is all (fig. 3). 
Alonopsis ambigua is exceptional in having an ephippium 
which might easily be mistaken for that of a Daphnid. 
In all cases in which an ephippium is formed provision 
must be made for casting it off with its contained eggs. 
This is effected by means of a “ line of weakness ” running 
round its edge. In the Daphnidae the chitin along this line 
is quite thin and unmarked by reticulations, but in the 
Lynceidae the line is marked by a series of elongated 
reticulations (fig. 4). The shell easily breaks along this line, 
and the ephippium is cast off, showing, in the Lynceidae, 
a jagged edge corresponding to the broken reticulations. 
The shell consists of two layers, of which the outer one only 
is thickened. The inner one, when the ephippium is cast 
off, wraps round the egg, while the edges of the outer one 
close together and prevent the evaporation of water from 
the cavity within. 
Generally the ephippium floats upon the surface and is 
easily transported by birds or other agencies. Probably the 
sudden appearance of species in a locality in which they have 
not previously been found is due to the transportation of 
the resting eggs. In a pond at Herringfleet^ kept continuously 
under observation, two rare species, Daphnia atkinsoni 
(fig. 2) and Alonopsis ambigua (the latter an addition to the 
British Fauna) appeared suddenly in this way and disappeared 
as suddenly — probably from the conditions proving un- 
suitable. 
In the Daphnidae there are generally two eggs in the 
ephippium, but never more. In the Lynceidae there is only 
one except in the case of Eurycereus lamellatus which has 
several. The Sididae and Polyphemidae have several resting 
eggs. 
