146 INVERTEBRATE ANIMALS. 
ORDERS CLADOCERA, COPEPODA, AND OSTRACODA.—These 
orders deserve mention more from the extreme abundance of 
their commoner forms than for any other reason, They include 
a number of minute Crustaceans, most of which are commonly 
called ‘“‘water-fleas,” and abound in fresh waters in Great 
Britain. They are, however, so small that, though visible to 
the naked eye, they can only be satisfactorily examined under 
the microscope. As an example of the Cladocera may be taken 
the “branched-horned water-flea” (Daphnia pulex, fig. 69, 6), 
thousands of which may be captured in any pond in summer. 
Sai 
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Fig. 69.—Fresh-water Entomostraca. a Cypris tris-striata; 6 Daphnia 
pulex ; ¢ Cyclops gquadricornis. 
In this pretty little species the whole body is enclosed in a bi- 
valve shell, which is so transparent that the whole organisation 
of the animal is clearly visible through it. The head is distinct, 
and carries a single eye. The greater antennz are branched. 
The males are smaller than the females, and much fewer in 
number; and it appears to be a well-established fact that the 
female, when once fertilised by the male, can not only lay eggs 
for the rest of her life, but can transmit the power of producing 
fertile ova to her young for several generations. Of the Cofe- 
poda one of the commonest is the Cyclops (fig. 69. ¢), in which 
the cephalothorax is covered by a shield, and there is a well- 
developed abdomen. The female carries on either side a kind 
of pouch or ovisac, in which the eggs remain till they are 
hatched. The little Ostvacoda (fig. 69, @). are all minute 
Crustaceans, which occur in both fresh and salt water. They 
are distinguished by the fact that the body is entirely enclosed 
