THE DECAPOD CRUSTACEA. 157 



(19) The last pair of swimmerets, or uropods, large and 

 powerful, with the telson constituting the tail-fan. 



Besides these nineteen pairs of appendages, we have the 

 large, compound, stalked eyes, which consist of a number of 

 eye-elements compacted together. 



As the thoracic appendages are removed, it will be found 

 that some of the gills come away with them. Break away 

 the gill-cover at the other side of the specimen you are 

 dissecting, and you will see that the gills lie in a chamber 

 opening freely to the surrounding water in front and behind. 

 In order that the lobster may breathe, it is necessary that 

 these gills be continually washed with fresh water. When 

 the lobster is swimming, or in a typical swimming Crustacean 

 like the prawn, this is accomplished by the movement of 

 the whole animal through the water ; but in a state of rest 

 the lobster would asphyxiate were it not that its second 

 maxillae are in constant movement, and by baling the water 

 out in front cause a constant current to pass in at the 

 posterior end of the gill-cover. This is readily seen in a 

 living Crustacean by suspending fine particles in the water 

 in which it is living, and is a point of great importance. 

 It is an advantage to the Crustacean to have its delicate 

 breathing organs protected by a gill-cover, but this advantage 

 brings with it the necessity for a mechanical means for 

 constantly renewing the water beneath the cover. In crabs 

 the protection of the gills is more efficiently provided for 

 than even in prawn and lobster, and they are less actively 

 motile animals than either. The result is that the renewal 

 of the water under the gill-cover of the crab has to be 

 provided for by active means, and many of the striking 

 peculiarities of the crab are associated with this fact. 



If you can obtain more than one specimen of Nephrops, 

 it is a good plan to dissect one, and then use the experience 

 gained to make a permanent preparation of another, laying 

 out the parts in order on a sheet of card or glass. The 

 flesh should be removed from the larger appendages, the 

 rings of the abdomen separated and cleaned, and the great 

 shield removed entire. During the process of preparation 

 you will find two skeletal parts which we have not yet 

 noticed the so-called internal skeleton of the thorax, and 

 the gizzard. The former is a very complex structure, formed 



