4IO CHARACTERS OF INVERTEBRATE ANIMALS 



Sub-class I. — Higher Crustacea (Malacostraca). 



Order i. Stalk-eyed Crustacea (Thoracostraca). — Lobster, Cray- 

 fish, Crab, Locust-Shrimp, Opossum-Shrimp, &c. 



Order 2. Sessile-eyed Crustacea (Arthrostraca). — Sand-Hop- 

 pers, Wood-Lice, &c. 



Order 3. Intermediate Crustacea (Leptostraca). — Mud-Shrimps 

 (Nebalia and its allies). 



Sub-class 2. — Lower Crustacea (Entomostraca). 



Order i. Barnacles (CiRRlPEDiA). — Ship-Barnacle, &c. 

 Order 2. Bivalve Crustacea (Ostracoda). — Cypris, &c. 

 Order 3. Fork-footed Crustacea (Copepoda). — Cyclops, Fish-Lice. 

 Order 4. Leaf-footed Crustacea (Phyllopoda). — Apus, Water- 

 Fleas, &c. 



Sub-class I. — Higher Crustacea (Malacostraca) 



The following features, illustrated by the Lobster, are char- 

 acteristic of the sub-class : — The body is made up of a constant 

 and limited number of segments, each of which, with the exception 

 of the last (or telson), bears a pair of appendages. In nearly all 

 cases the segments are twenty in number, distributed as follows : — 

 head, 5 ; thorax, 8 ; Abdomen, 7. The excretory organs typically 

 present are antennary glands. The development is complex, 

 and in most cases there is a larval form, differing markedly in. 

 appearance from the adult. 



Order i. — Stalk-eyed CRUSTACEA (Thoracostraca) 



In this order the head and thorax are fused together, and the 

 eyes are usually situated on stalks. There are four sub-orders: — 

 I. Ten-legged Crustacea [Decapoda), 2. Opossum- Shrimps 

 {Sckisopoda), 3. Locust- Shrimps {Stomatopoda), and 4. the 

 Cumacea. 



I. The Decapoda are so named because the last five pairs of 

 thoracic limbs are seven-jointed locomotor organs, in which the 

 outer branches are absent in the adult. 



One large section of the sub -order is distinguished by the 

 presence of a long and powerful tail, provided with a well- 

 developed tail-fin. Among the British species here included are 

 the following: — Lobster {Jiomarus vulgaris) (fig. 250), Rock- 



