74 INTRODUCTION TO ZOOLOGY. 



beautiful provision exists, by which it is from time to time 

 thrown off, and its place supplied bjone of larger dimensions. 

 In two or three days, the new covering assumes the hardness 

 of the old one; and, until then, the animal, as if conscious of 

 its defenceless state, avoids, as much as possible, all exposure. 

 We shall revert to this subject in treating of the best known 

 native species. 



Eeproduction. All of them possess the capability of repro- 

 ducing extremities which are injured. Thus, if the leg of a 

 Crab be fractured, it throws off the injured limb, near to the 

 body. "It has the power of doing so apparently for two 

 purposes to save ;the excessive flow of blood which always 

 takes place at the first wound, and to lay bare the organ which 

 is to reproduce the future limb. * As soon as the injured limb 

 has been thrown off, the bleeding stops ; but if the animal is 

 unable, from weakness or any other cause, to effect this, tho 

 result is fatal. The growth of the new limb is slow, until 

 after the period of the next moult, when it rapidly assumes 

 its full proportions. " 



Bespiration. Every one who has opened the "shell" of 

 the common Crab, has noticed a number of leaf-like organs, 

 regularly arranged in two parcels, with the points of the 

 little leaves or plates in each parcel brought nearly to- 

 gether (Fig. 47). These are the branchiae or gills, organs 

 admirably adapted to the aquatic life of the animal. In the 

 Lobster the arrangement of the parts is different (Fig. 48), 

 being accommodated to the different form of the body, but 

 providing no less effectually for the seration of the circulating 

 fluid. In other Crustacea, the gills are formed like feathery 

 tufts, and float freely in the water (Fig. 49) ; while, in one 



Fig. 49. SQUILLA. 



* H. D. S. Goodsir, on "the Mode of Reproduction of Lost Parts in the 

 Crustacea." Anatomical and Pathological Observations. Edinburgh, 1845. 



