562 



CRUSTACEA. 



Locust-shrimps (Souilla), all of which are inhabitants of the sea. 

 The Crustaceans of this order (fig. 423) have a short cephalothoracic 



carapace, which does not protect the hinder 

 segments of the thorax. There are five 

 pairs of maxillipedes, and three pairs of 

 thoracic legs. The branchice are not en- 

 closed in a cephalothoracic gill-chamber 

 on each side, but are in the form of tufts 

 attached to the abdominal feet. The eyes 

 and antenna, are attached to a somite 

 which is not soldered to the cephalo- 

 thorax. 



As regards their distribution in time, 

 it is doubtful if any Palaeozoic types of 

 the Crustacea can be referred to this 

 order, but the genera Necroscilla and 

 Diplostylus, of the Coal-measures, may 

 possibly be ancient representatives of 

 the Stomatopoda. In the Jurassic rocks 

 (Lithographic Slates) of Germany is 

 found the genus Sculda, which is re- 

 lated to the recent genus Squilla, and 

 species of the latter have been de- 

 scribed from the Cretaceous rocks, 

 while others occur in the Eocene 

 Tertiary. 

 Order IV. Decapoda. — This order includes the Shrimps and 

 Lobsters, the Hermit-crabs, and the true Crabs, and comprises the 

 most highly organised of all the Crustacea. Most of the Decapods 

 are aquatic in their habits, and they are usually protected by strong 

 resisting shells. There is always a complicated set of "gnathites," 

 or appendages modified for masticatory purposes, surrounding the 

 mouth. The ambulatory feet are made up of five pairs of legs 

 (hence the name of the order) ; the first pair — and often some 

 other pairs behind this — being "chelate" or having their extremities 

 developed into nipping-clatvs. The branchice are pyramidal, and are 

 contained in cavities at the side of the thorax. The carapace is 

 well developed, and covers all the segments of the head and 

 thorax. 



The Decapoda are divided into three tribes, termed respectively 

 the Macrura, Anomura, and Brachyura, and characterised by the 

 nature of the abdomen. 



Tribe A. Macrura. — This tribe includes the " Long-tailed 

 Decapods," such as the Lobsters, Cray-fish, Shrimps, and Prawns, 

 in which the abdomen is well developed, often longer than the 



Fig. 423. — Squilla mantis, the Locust- 

 shrimp. 



