3252 THE JOINTED ANIMALS 



from the posterior end of the body are the egg sacs. The appendages of the 

 head and thorax are more or less reduced, being either absent or converted into 

 adhesive hoops or suckers. 



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 inclosing the greater part of the body, this carapace being an 

 extension of the dorsal surface of the head segments. An example of the order is 

 the water flea (Daphnia pulex), to which Acanthocercus, represented in the 

 figure on p. 3251, 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 setae, from which the genus 

 takes its name. At the front of the head (A) is a large compound eye (O), 

 and the branched and plumed appendages projecting from beneath the sides of 

 the head are antennae (R, T). The first pair of antennae are small and simple. 

 The jaws consist of the mandibles and the first pair of maxillae, the second 



pair of maxillae being obsolete in the adult. The 

 thorax comprises five segments, each bearing a pair 

 of leaf-like swimming limbs. The abdomen consists 

 of three segments and is limbless. The males of 

 Acanthocercus are smaller than the females and 

 much rarer, being generally met with in the 

 autumn. Eggs are laid both in summer and winter 

 -and are passed into a brood pouch, separating the 

 upper surface of the thorax from the backward 

 EGG CAPSUUS OR Ephippium OF extension of fa cara pace. Here the summer eggs 

 WATER FI.EA, Acanthocercus . t, * i. u .. *t, * i j 1- j c 



(Much enlarged.) hatch, but the winter set are inclosed in a kind of 



capsule developed from part of the carapace. This 



capsule, called the ephippiitm, is cast off with the next molt of the mother's integu- 

 ment, and falling to the bottom of the water gives exit to the embryos, which hatch 

 in its interior. Another type is the glassy Leptodora hyalina, so called on account 

 of its semitransparency, which inhabits the open water of fresh- water lakes. The 

 shell is so much reduced as scarcely to envelop the animal. 



LEAF-FOOTED GROUP Order PHYLLOPODA 



Some of the members of this group are relatively large, the body being long 

 and composed of a great number of segments, of w r hich the thoracic, and sometimes 

 the abdominal, are furnished with leaf-like gill-bearing appendages. 



In the family Apodidce, containing the genera Apus and Lepidurus, the anterior 

 end of the body is covered with a carapace, projecting from the head over the free 



