228 MANUAL OF ZOOLOGY. 



The Decapoda are divided into three tribes, termed respec- 

 tively the Macrura, Anomura, and Brachyura, and character- 

 ised by the nature of the abdomen. 



Tribe A. Macrura. — The " long-tailed" Decapods included 

 in this tribe are distinguished by the possession of a well-de- 

 veloped abdomen, often longer than the cephalothorax, the 

 posterior extremity of which forms a powerful natatory organ 

 or caudal fin. As regards the development of the Macrura, 

 most appear at first in the form of " Zoese ; * but there is little 

 metamorphosis in the common Lobster, and there is said to be 

 none in the Cray-fish {Astacus fluviatilis) and in one of the 

 Land -crabs {Gecarcinus). Fritz Miiller, again, has shown 

 that the primitive form of one of the Shrimps {Peneus) is that 

 of a " nauplius." This section comprises the Lobster, Cray- 

 fish, Shrimp, Prawn, &c., of which the Lobster may be taken as 

 the type. 



In the Lobster (fig. 74) the somites of the head and thorax 

 are amalgamated into a single mass, the " cephalothorax," 

 covered by a carapace or shield, which is developed from "the 

 lateral or epimeral elements of the fourth cephalic ring, which 

 meet along the back, and give way preparatory to the moult. 

 The tergal elements of the thoracic rings are not developed in 

 either Crabs or Lobsters ; when these rings are exposed by 

 lifting up the cephalothoracic shield, the epimeral parts alone 

 are seen, converging obliquely towards one another, but not 

 joined at their apices." — -(Owen.) 



The first segment of the head bears the compound eyes, 

 which are supported upon long and movable eye-stalks or 

 peduncles. Behind these come two pairs of jointed tactile 

 organs, the larger called the "great antennse" (fig. 74 ^a), the 

 smaller the " antennules" (a). The mouth is situated on the 

 under surface of the front of the head, and is provided from 

 before backwards with an upper lip (" labrum"), two " mandi- 

 bles," two pairs of " maxillae," three pairs of " maxillipedes" 



* The young Decapod, in most cases, leaves the egg in a larval form so 

 different to the adult that it was originally described as a distinct animal 

 under the name of Zoea. In this stage (fig. 76) the thoracic segments 

 with the five pairs of legs proper to the adult are either wanting or are 

 quite rudimentary. The abdomen and tail are without appendages, and 

 the lat .er is composed of a single piece. The foot-jaws are in the form < I 

 natatory forked feet, and the mandible has no palp. Lastly, there are no 

 branchiie, and respiration is carried on by the lateral parts of the carapace. 

 The "Zoea" is separated from the "nauplius" by having a segmented 

 body, large paired eyes (sometimes with a median eye), and a carapace. 

 The form proper to the adult is not attained until after several moults, 

 constituting a genuine metamorphosis, though one which is effected byvtiy 

 gradual stages. 



