284 
CR U S TA CE A NS. 
of the head and thorax are more or less reduced, being either absent or converted 
into adhesive hoops or suckers. 
FISH-LICE. 
a, Lerneeonema (enlarged 3 times); b, Brachiella (enlarged 9 times); c, Pennella (enlarged 5 times); 
d, Hccmobaphes (nat. size); e, Caligus (enlarged 3 times). 
Order Cladocera. 
The members of this order take their names from the large and branched 
antennae, which serve as swimming organs. They are all small, and the carapace 
forms a bivalve shell enclosing the greater part of the body, this carapace being an 
extension of the dorsal surface of the head-segments. A11 example of the order is 
the water-flea (.Daphnia pulex), to which Acanthocercus, represented in the 
figure on p. 285 , is nearly allied. Here the body is narrowed in front, and at 
the posterior end, where the carapace (S) is deeply notched, is the tip of the 
abdomen (C), bearing the pair of rigid barbed setse, from which the genus 
