40 OCEAN LIFE. 



contains a considerable portion of carbonate of lime. In many of 

 the smaller forms, as in the Shrimps and Prawns, it takes the 

 consistence of thin transparent horn ; but in the larger, as in 

 Lobsters and Crabs, it acquires a great density, is perfectly 

 opaque, and of the hardness of true shell or even of stone. In 

 the tips of the stout claws of the Crab, we see it at its maximum 

 of induration. 



" The periodic casting of the skin is a needful provision for 

 growth in these animals, as in Insects ; in them it is confined to 

 the caterpillar state, in which alone growth takes place ; but here 

 it prevails, because growth is continued long after the perfect 

 form is attained. The rigidity of the encasing armor forbids 

 the possibility of increase in its capacity. The growth, there- 

 fore, is periodic. At certain intervals the hard crust is thrown 

 off in several pieces, a new crust having been prepared beneath, 

 which is at first soft, flexible, and expansible. The body, now 

 freed, instantly enlarges in all directions, and, in a few minutes, 

 has attained the full extent of growth needed. The crust at 

 once hardens, and in a brief space becomes as inflexible as was 

 its predecessor, admitting no further enlargement either of its 

 own surface or of the contained organs. The animal usually un- 

 dergoes this process in the most retired situation it can find, in- 

 stinctively conscious of its unprotected position when soft, and 

 apparently feeling sick and feeble. 



"Another interesting circumstance in the economy of this 

 class, is the power of renewing injured or lost limbs. If one of 

 the joints of a Crab's legs be wounded, it would bleed to death 

 but for this provision. The animal, however, stiffens the hurt 

 limb, and suddenly throws it off, the separation invariably taking 

 place at the point where the second joint is united with the first. 

 A small gland is placed here, according to Mr. H. Goodsir, which 

 supplies material for future legs as required. ' When the limb is 

 thrown off, the blood-vessels and nerve retract, thus leaving a 

 small cavity in the new-made surface. It is from this cavity that 

 the germ of the future leg springs. A scar forms over the raw 

 surface caused by the separation, which afterwards forms a sheath 

 for the young leg.' 'As the growth advances, the shape of the 

 new member becomes apparent, and constrictions appear, indi- 



