CRUSTACEA. 129 



is so complex that it will be as well to omit altogether the 

 less important orders, merely giving the names and leading 

 characters of these in an appendix. It has also been thought 

 advisable to invert the usual order here adopted, and to com- 

 mence with the consideration of the highest sections of the 

 class first. 



OKDER DECAPODA. The Crustacea included in this order 

 derive their name from the fact that they all possess five pairs 

 of legs (Gr. deka^ ten ; podes, feet). They belong to a large 

 section known as the " stalk-eyed " Crustaceans, from the fact 

 that the eyes are supported by long, movable stalks. They 

 include the lobsters, shrimps, cray-fish, crabs, hermit-crabs, and 

 other forms, and are the most highly organized and most familiar 

 of the whole class of the Crustacea. They are divided into 

 three very well marked groups or tribes, all of which can be 

 exemplified by the well-known British species. 



A. Macrura. The name of Macrura (Gr. makros, long; 

 and owra, tail) is given to those ten-footed Crustaceans which 

 have a long and well-developed tail. Among these are the 

 lobster, shrimp, prawn, and cray-fish, of which the lobster may 

 be selected as a good typical example. 



In the lobster (Fig. 50) the body is at once seen to be 

 composed of two parts, familiarly called the "head" and 

 " tail." The so-called head is covered by a great shield termed 

 the "carapace" (Fig. 50, ca), and it is in reality the cephalo- 

 thorax, being composed of the amalgamated segments which 

 belong to the true head and to the thorax. The so-called 

 tail is really the abdomen, and it is composed of a number of 

 segments which are not immovably united together, as in the 

 cephalo-thorax, but are movably jointed together. The vari- 

 ous appendages of the* animal are arranged in pairs on the 

 under surface of the body ; and, where the segments are com- 

 pletely amalgamated (as in the cephalo-thorax), their existence 

 may, nevertheless, be determined by the presence of a pair of 

 appendages. The first segment of the head carries a pair of 

 compound eyes, made up of a number of simple lenses aggre- 

 gated together, and supported upon long and movable eye- 

 stalks. Behind these come two pairs of jointed organs of 

 touch, which are known as the " antennas." The front pair is 

 much smaller than the hinder pair, and they are known re- 

 spectively as the "lesser antennas," or " antennules," and 

 the " great antennas." Behind these, again, comes the mouth, 

 which is placed on the under surface of the head, and is pro- 



