240 The Study of Animal Life part m 



segments and appendages varies greatly. The little larva v^hich 

 hatches from the egg is usually a " Nauplius " — an unsegmented 

 creature with only three pairs of appendages and a median eye. 



The brine - shrimps (Artemia), the related genus Branchipus, 

 the old-fashioned freshwater Apus ; the common water-flea Daphnia 

 and its relatives, like Leptodora and Moina, are united in the order 

 of Phyllopods. 



The 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. Chondracanthus, Caligus, 

 Lerticsd) are known as Copepods. The free-swimming forms often 

 occur in great swarms and are devoured by fishes. 



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

 (Lepas) pendent from floating "timber," and the Aege-aeizX^Sacculina 

 under the tail of crabs, represent the order Cirripedia. 



The higher Crustaceans are grouped together as Malacostraca. 

 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 Zoea, the lobster in a yet higher 

 form, and the crayfish as a miniature adult. 



Simplest of these higher Crustaceatis, 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-louse Oniscus has counterparts 

 {AseSus, Idotea) on the shore, and several remarkable parasitic 

 relatives. Among the liighest forms are the long-tailed lobsters 

 (Homarus, Palimirus), and crayfishes {Astacus), and shrimps 

 (Crangon), and prawms (Palamon, Pandalus) ; the soft -tailed 

 hermit crabs (Pagurus) ; and the short tailed crabs (e.g. Cancer, 

 Carcinus, Dromia). 



(i) Frotracliea/ta. — Peripatus. This remarkable genus, repre- 



FiG. 46. — Peripatus. (From Chambers's Encyclop. ; after Moseley.) 



sented by about a dozen widely-distributed species, seems to be a 

 survivor of the ancestral insects. Worm-like or caterpillar-like in 



