Xll INTRODUCTION. 



In tlie course of its development from the egg, the Prawn 

 passes through two remarkable stages before gaining the adult 

 form. In the first of those stages the larva assumes the form of 

 an oval body with a median frontal eye, a large labrum and three 

 pairs of setose swimming appendages. In the latter it has a 

 rounded carapace, antennules, antennae, and mandibles, with four 

 other pairs of appendages of which the last three are bifurcated, 

 and five pairs of short lamellar processes ; the abdomen has long 

 and distinct segments, and ends in two setose processes, but is 

 destitute of appendages. The first of these is called the Nauplius, 

 and the second the Zooea stage. 



The Prawn has the majority of the features thus briefly 

 sketched in common with the Crayfishes, the Crawfishes or 

 Rock-Lobsters, the Shrimps, and other forms of Crustacea 

 belonging to the great division of the Macroura or long-tailed 

 Decapods. Though differing considerably from one another in 

 the form of the appendages and the arrangement and structure 

 of their gills, the members of these different groups all agree in 

 the number and relations of the segments of the head, thorax, 

 and abdomen, in the general character of the appendages with 

 which these segments are severally provided, and in the relative 

 position and general structure of the digestive, nervous, circu- 

 latory and reproductive organs. 



Nearly related to these, but with some marked points of 

 difference, are the Crabs, Brachyura or Short-tailed Decapods. 

 If we examine any of the common shore-crabs, we find the 

 general shape of the body very different from that of the Prawns. 

 We find, as in the Prawn, a broad cephalo-thoracic shield or 

 carapace, but this, instead of merely occupying the anterior 

 portion of the body of the animal, seems to cover the whole 

 upper surface with a hard shell. The reason of this appearance 

 is that in the Crab, though the total number of segments in the 

 body is the same as in the Prawn, the whole of the abdomen is 

 in an extremely rudimentary condition, and is tucked in under 

 the thorax so as to be quite concealed when the animal is looked 

 at from above. Though the abdominal segments, except in cases 

 in which several coalesce in the full grown animal, are the same 

 in number in the Crab as in the Prawn, there are no swimmerets 

 appended to them ; in the female, however, the four anterior 



