INTRODUCTION. 



The Crustacea are a group of back-boneless or invertebrate 

 animals, distinguished by the possession of a more or less hard 

 jointed external crust or exoskeleton, nearly all living habitually 

 in water, and breathing by means of gills. 



Of the Crustacea the large group of the Malacostraca are 

 distinguished from the rest (the Entomostraca) by the number 

 of the segments of the body, of which there are always twenty, 

 except in certain cases in which amalgamation or abortion of 

 one or more of the segments has taken place. 



Taking as a readily obtainable example of the Malacostracous 

 Crustacea the common prawn* (Penmus canaliculatus of Milne- 

 Edwards), we find that it consists of two clearly distinguishable 

 parts — an anterior shield-like portion bearing the eyes and 

 feelers and extends back over about a third of the length 

 of the body, and a posterior portion, distinguished by being 

 divided by transverse lines into rings or segments which are 

 found to be freely moveable on one another, so as to confer a 

 considerable degree of flexibility on this posterior region of the 

 body. When one of these rings or segments is examined 

 separately, it is found to consist above of a high convex dorsal 

 arch, and below of a narrow straight ventral bar ; on either side 

 the dorsal arch sends down a free fold or flap, so that the ventral 

 bar is situated in a deep longitudinal hollow or groove enclosed 

 between these. The dorsal arch of each segment overlaps to a 

 considerable extent that following it ; the ventral bars are 

 separated from one another by wide spaces occupied by a thin 

 translucent membrane — the sternal membrane. The dorsal arch 

 is called the tergvm, the ventral bar the sternum, the side flaps 



* The term Prawn is very generally applied in Australia to species of the 

 genus Penceus, the genus Paleemon being less widely known as a article of diet. 



