CHAP, xii BACKBONELESS ANIMALS 251 



cldpiis, the old-fashioned freshwater Apus ; the common 

 water-flea Daphnia and its relatives, like Leptodora and 

 Moina, are united in the order of Phyllopods. 



Other small " water-fleas," of which Cypris is a very 

 common representative, and which are very abundant 

 in sea and lake, form the order of Ostracods. 



Another " water-flea ' ' Cyclops and many more or less 

 degenerate " fish-lice ' and other ectoparasites (e.g. 

 Chondr acanthus, Caligus, Lerncea) are known as Copepods. 

 The free-swimming forms often occur in great swarms 

 and are devoured by fishes, thus playing an important 

 part in the economy of the waters. 



The acorn-shells (Balanus) crusting the rocks, the 

 barnacles (Lepas) pendent from floating "timber," and 

 the degenerate Sacculina under the tail of crabs, repre- 

 sent the order Cirripedia. 



The higher Crustaceans are grouped together as Mala- 

 costraca. The body usually consists of nineteen segments, 

 five forming the head, eight the thorax, six the abdomen 

 or tail. In most cases the larva is hatched at a higher 

 level of structure than the Nauplius represents, but the 

 shrimp-like Penceus begins life as a Nauplius while the 

 crab is hatched as a Zosea, the lobster in a yet higher 

 form, and the crayfish as a miniature adult (fig. 111). 



Simplest of these higher Crustaceans, in some ways 

 like a survivor of their hypothetical ancestors, is the 

 marine genus Nebalia, but we are more familiar with the 

 Amphipods (e.g. Gammarus) which jerk themselves along 

 sideways or shelter under stones both in fresh and 

 salt water. The wood-lice (Oniscus, Porcellio, etc.) have 

 counterparts (Asellus, Idotea) on the shore, and several 

 remarkable parasitic relatives. Among the highest 

 forms are the long-tailed lobsters (Homarns, Palinurus), 

 and crayfishes (Astcicus), and shrimps (Crangon), and 

 prawns (Palccmon, Pandalus); the soft-tailed hermit 

 crabs (Pagunis) ; and the short-tailed crabs (e.g. Cancer, 

 Carcinus, Dromia). 



Onychophora or Prototracheata.- -This class includes a 

 few archaic types, which like other old-fashioned crea- 

 tures, are very widely distributed, e.g. in Tropical Africa, 



