192 MANUAL OP ZOOLOGY. 



sides of the thorax. The carapace is large, covering the head 

 and*thorax and the anterior part of the* abdomen. 



The Decapoda are divided into three tribes, termed respec- 

 tively the Mavrwra, Anomura and Brachyura, and characterised 

 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 

 developed abdomen, often longer than the cephalothorax, the 

 posterior extremity of which forms a powerful natatory organ 

 or caudal fin. This section comprises the Lobster, Cray-fish, 

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

 the type. 



In the Lobster (Jig. 49) 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 antenna3,' the smaller the 

 ' antennules.' 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 'mandibles,' two pairs of 

 ' maxilla,' three pairs of ' maxillipedes ' or ' foot-jaws,' and a 

 bifid lower lip, or ' metastoma.' The five remaining segments 

 of the thorax carry the five pairs of ambulatory legs, of which 

 the first constitute the great claws, or ' chelaB ; ' the next two 

 pairs are also chelate, though much smaller, and the last two 

 pairs are terminated by simply pointed extremities. The seg- 

 ments of the abdomen carry each a pair of natatory limbs, or 

 ' swimmerets,' the last pair being greatly expanded, and con- 

 stituting, with the 'telson,' a powerful caudal fin. Most pos- 

 teriorly of all is the post-anal plate, or ' telson,' which may be 

 looked upon either as an azygos appendage, or as a terminal 

 segment, which has no lateral appendages. 



The mouth leads by a short oesophagus into a globose 

 stomach, in the cardiac portion of which is a calcareous appa- 

 ratus for triturating the food, which is commonly called the 

 * lady in the lobster.' The intestine is continued backwards 

 from the stomach without convolutions, and the anal aperture 



