MALACOSTRACA 26 I 



561. Order 6.— The Malacoslraca are a large group, comprising all the 

 larger Crustacea. The head and thorax together are composed of thirteen 

 segments and the abdomen of six. There are always two pairs of antennae, 

 a pair of mandibles and two pairs of maxilla?. In most cases several more 

 pairs of appendages function as mouth parts. This order is very large 

 and includes shrimps, prawns, crayfish, lobsters, crabs, sand fleas and 

 "wood-lice." Of the five great divisions of the group the following are 

 the most important. Legion 2. Thoracostraca. The compound eyes are 

 on movable stalks and the head and most or all of the segments of the 

 thorax are covered by a single cuticular shield. The gills are usually 

 attached to the basal joints of the thoracic appendages or to the adjacent 

 parts of the body- wall and are covered by the cephalo-thoracic shield. 



Fig. 150. — Caprella geometrica, an Amphipod. (From Galloway, after Vcrrill.) 



The sub-order Decapoda comprises the most important families. In this 

 group the five pairs of appendages from the 9th to 13th segments inclusive 

 are ambulatory appendages. Those of the 3rd to 8th are mouth parts. 

 The following analysis of the classes of the malacostraca will show the 

 relation of the groups mentioned. 



Order, Malacostraca. 



Legion i. Leptostraca. 

 Legion 2. Thoracostraca. 



Sub-order i. Schizopoda. \ 



Sub-order 2. Decapoda. 



Section i. Macrura natantia, shrimps, pawns. 

 Section 2. Macrura reptantia, crayfish, lobster. 

 Section 3. Anomura, hermit crabs. 

 Section 4. Brachyura, crabs. 

 Sub-order 3. Cumacea. 

 Legion 3. Stomatopoda. 

 Legion 4. Anomostraca. 

 Legion 5. Arthrostraca. 



Sub-order i. Anisopoda. 



Sub-order 2. Isopoda, "pill bug" = " wood-louse." 



Sub-order 3. Amphipoda, sand fleas. 



