38 
BULLETIN OF THE LABORATORIES 
are graceful in movements and slender in form. Very generally they 
are more or less boat-shaped and the polished shell is modeled like a 
clipper, the head-shield forming the prow, while, in a few cases of the 
best swimmers, (as Carnptocercus^ Acroperus^ and Alonopsis) there 
is developed a sharp ridge upon the back which bears an unmistakable 
resemblance to the keel of a boat and, since the animals swim upon 
the back, may not impossibly serve a similar purpose. 
In the mud-loving species, on the contrary, the body is clumsy and 
approaches the spheroid in form, the antennae, which normally are oar- 
like and long, bearing fringed setae, are short and are armed with claw- 
like spines and smooth setae. The post-abdomen or tail, which in the 
natatory species is reduced in size and subordinated in function, is here 
enlarged and armed with numerous and considerable spines. The 
front of the head is either reduced and pointed, or, if there be a long 
beak, it is turned up out of the way. The meaning of all this is that 
the animal no longer swims on its back, but creeps humbly and prone 
and requires the efforts of post-abdomen and antennae, as pushing poles 
to make its way through the debris in seach of food. The effect of 
this manner of life is seen in several other ways, as in the structure of 
the shell itself. In several of our species the shell, which is renewed 
periodically, is not, as in other cladocera, moulted, but remains as a 
sort of outgrown overcoat, after the new shell has become perfected. 
The result of this is what might be expected from the sluggish na- 
ture of the animal, the supply of clothing thus accumulated becomes 
so onerous a burden that it no longer could swim if it desired and is 
thus fettered to the life it early chose. While this is true of a few 
only, yet in all the limicole cladocera the cuticle becomes indurated 
and it follows that respiration, which normally takes place from the en- 
tire surface of the body, becomes restricted to those membranes in 
contact with the water within the valves of the shell. As a partial 
offset to this disadvantage, the strictly respiratory appendages on the 
feet are enlarged more than in most groups. The sensory organs are 
modified in several ways. Kurz calls attention to the fact that the 
antennules are movably joined to the body in the limicole cladocera ; 
they are also, as a rule, rather large and well endowed with sensory 
apparatus. The compound eye is small and, in one species, as we 
shall see, fails to develop at all. 
Monospilus dispar, Sars. is the most remarkable of all filth-dwel- 
lers. This animal may probably lay claim to be called the rarest of 
