244 ANNULATA. 
Sus-Ciass II.—Ma.Lacostraca. 
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 antennze 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 (Oxzscus) are good examples. 
The Decapoda form the most important order of Madacos- 
traca. The head and thorax are enveloped in a carapace 
and there are five pairs of legs (including chele). The 
eyes are stalked. They include the lobsters, shrimps and 
prawns, the crabs and hermit-crabs. 
The crabs have the 
Fig. 161.—A Z@a Larva OF A’ abdomen reduced and 
Decapop. (Lateral view.) tucked forward on the 
under side of the thorax. 
The appendages are 
closely similar to those 
of the lobsters, but the 
nerve-ganglia are more 
consolidated. The her- 
WNW ( mit-crabs have a long, 
OY Wh) em Componne soft abdomen, which 
Jd they protect in a shell. 
The shell is usually a 
disused whelk-shell or 
that of some smaller 
gastropod. The chelz 
Rostrum: are of different sizes, 
adapted to the spiral of 
the shell. The appen- 
Note the paired eyes, the spines, abdomen without dages on the abdomen 
appendages, and gills with no gill-cover. are ves tigial. 
Dorsal Spine. 
Abdomen. 
Crass II.—PROTRACHEATA. 
Peripatus constitutes, not only the type, but the sole 
order of this class. 
