244 



ANNUL AT A, 



Sub-Class II. — Malacostraca. 



The Malacostraca include the higher types of Crustacea. 

 The body usually consists of twenty segments and the 

 appendages are much modified. The excretory organ, the 

 antennary gland, opens on the second antennae and there 

 is usually a gastric mill. The nauplius larva is of rare 

 occurrence, the early development being embryonic. 



The order Arthrostraca comprises Crustacea with sessile 

 eyes, and with not more than two thoracic segments fused 

 with the head. The freshwater shrimps, sand-hoppers, and 

 the terrestrial woodlouse {Oniscus) are good examples. 

 The Decapoda form the most important order oi Malacos- 

 traca, The head and thorax are enveloped in a carapace 

 and there are five pairs of legs (including chelae). The 

 eyes are stalked. They include the lobsters, shrimps and 

 prawns, the crabs and hermit-crabs. 



The crabs have the 



Fig. i6i. — A ZcEA Larva of a 

 Decapod. (Lateral view.) 



Dorsal Spine. 



abdomen reduced and 

 tucked forward on the 

 under side of the thorax. 

 The appendages are 

 closely similar to those 

 of the lobsters, but thf 

 nerve-ganglia are more 

 consolidated. The her 

 mit-crabs have a long 

 soft abdomen, whicl 

 they protect in a shell 

 The shell is usually 

 disused whelk- shell o 

 that of some smalle 

 gastropod. The chek 

 are of different size^i 

 adapted to the spiral c 

 the shell. The apper 



Note the paired eyes, the spines, abdomen without (Jg^prgg qj^ \)c\^ abdomC 

 J _.-j „:n :.T :ii & ... 



are vestigial. 



Abdomen. 



Compound 

 Eyes. 



Rostrum, 



appendages, and gills with no gill-cover. 



Class II. — Protracheata. 

 Peripatus constitutes, not only the type, but the so 

 order of this class. 



