PHYLUM ARTHROPODA 



305 



night. They are remarkable for their Hght organs and for the amplitude 

 of their daily migration. Schools of them are a food for the filtering 

 whales. 



The Isopoda are dorsoventrally flattened crustaceans without cara- 

 paces. They are found in both salt and fresh water. This order also in- 

 cludes the only truly terrestrial crustaceans, the pill-bugs and sow-bugs 

 (Fig. 16.14). The Amphipoda (shown on Fig. 16.2) also lack carapaces, 

 but they are compressed laterally rather than dorsoventrally. They are 

 common in all waters, forming an important fish food. Finally, the order 

 Decapoda includes a variety of familiar forms such as shrimps, crabs and 

 lobsters. 



139. The Subphylum Labiata 



All labiates have a distinct head enclosed in a head capsule, which 

 usually bears eyes, a pair of many-jointed antennae, mandibles, maxil- 

 lae, and a labium formed by the embryonic fusion of the secontl max- 

 illae. Trunk appendages are uniramous and usually seven jointed, 

 ending in terminal claws. The subphylum can be divided into two 

 superclasses, the Myriapoda in which most of the body segments have 

 walking legs, and the Hexapoda in which only the first three body 

 segments have walking legs. 



Myriapods are simpler and less specialized. They lack compound 

 eyes, having instead aggregates of ommatidia clustered on the sides of 

 the head. The body segments are similar to one another like those of 

 the annelids. Behavior patterns are simjjle. 



Of the myriapods, centipedes and millipedes are the only familiar 

 groups. Centipedes, class Chilopoda (Fig. 16.15 A), are predaceous ani- 

 mals hunting down insects and killing them with their poison claws, 

 which are the modified legs of the first body segment. Each of the re- 

 maining body segments except the last has a pair of long walking legs. 

 The total number of legs ranges from 15 to 173 pairs in different species. 





B 



Figure 16.15. Examples of the Myriapoda. A, Order Chilopoda, the centipedes. B, 

 Order Diplopoda, the millipedes. (Villee: Biology.) 



