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Of the small long-tailed Crustacea that often live in the Aqua- 

 rium we will mention the delicate , often transparent praicns 

 and slirimps, such as the Palaemon, Crangon, Alpheus, 

 Peneus , Lysmata , and others , which are remarkable 

 for their merry way of jumping. The first named inhabits all 

 rocky coasts in troops, and is food for innumerable other crea- 

 tures , being chiefly used in this way in the Aquarium. The- 

 motion of these prawns in walking or swimming is very light 

 and graceful , and their sensitiveness so great, that they feel 

 the slightest shaking of the water, and respond to it by immense 

 leaps. They are also sensitive to sound, as is proved by expe- 

 riment ; their organ of hearing is situated at the base of the 

 hinder feelers, and consists in a little bag lined with fine au- 

 ditory hairs , and opening outwardly in a slit. The waves of 

 sound cause these fine hairs to vibrate, and the motion is com- 

 municated to the auditory nerve. But to strengthen the sound 

 the ear-sack contains so called auditory stones , concretions 

 which are often lost , and then the animal replaces them by 

 little grains of sand, which it sticks into its ears! 



A rare creature among shrimps is the Stenopus spinosus, 

 an extremely delicate and pleasing animal, which may now and 

 then be found in a separate tank in the Aquarium. 



We will now turn to a curiously developed crustacean, which 

 may be called an intermediate member between the long-tailed 

 Crustacea just described and the Brachyura or short-tailed 

 crabs. It is the Hermit-crab (Eupagurus). 



A whelk-shell running on the legs of a crab, and carrying 

 sea-anemones on its back such is the first impression made on 

 any person looking at a hermit-crab for the first time. 



The droll figure , which seems composed of parts of three 

 different animals , at once interests us and excites a wish to 

 learn more about it. The matter is simpler than it seems, and 

 is explained as follows. Hermit-crabs are animals who take up 

 their quarters in the empty shells of Mollusks, gathered at the 

 bottom of the sea, and chosen according to the size, that is, 

 the age, of the crabs which intend to inhabit them. When the 

 crab has chosen his house, he inserts himself into it backwards, 

 and fastens himself firmly by the hind part of his body. This 

 hind part has become , in the course of many thousands of 

 centuries of adaptation to this mode of life, altogether unsym- 

 metrical and soft, and resembles a long bag furnished with 

 a pair of little hooks at the end for sticking fast in the 

 shell , and by this means the animal adheres so firmly to- 



