The Museum Gazette 



crustacean of the Macruran or long-tailed type, such as the 

 lobster, &c, only it is broader in proportion. It has ex- 

 changed its swimming feet for claws and walking legs, and 

 is now of the form termed a Megalope. But it still swims in 

 the open, only it begins to frequent the bottom a little. 

 Finally it tucks its tail beneath the body and assumes the 

 Brachyuran or short-tailed true 'crab' form (see illustration). 

 Its size during these changes ranges from a small ' o ' to a 

 capital ' O ' in this type." 



How a Crab Changes its Shell. 



" While encased in its firm shell a crab is not seen to grow, 

 the shell is a ' fixed quantity,' but the crab is growing within, 

 in a state of compression, until, by-and-by, the situation 

 becomes awkward and something must be done. The shell 

 opens at the joint between the carapace and the abdomen or 

 1 tail,' and the crab, covered only with a leathery skin, and 

 elastic as a bag of jelly, withdraws from its tight jacket and 

 is about one-third larger than this old garment. It seeks a 

 place of shelter, for it is aware of its vulnerability, and 

 remains there, still growing, for a day or two, until the cuticle 

 of its skin shall have secreted lime and become a casing in 

 which its owner can face the dangers of the open world once 

 more. This ecdysis (or change of shell) presents many 

 interesting points. Not only is the external and visible 

 portion — shell of back, tail, legs, antennae and eyes — cast off, 

 but with this, and all without severance, the internal shelly 

 parts as well. The covering of the gills, the complicated 

 honeycomb-like structure termed the ' endophragmal system,' 

 the lining of the stomach with its internal ' teeth,' and the 

 blade-like processes which act as tendons in each joint — all 

 are cast, and in the same position as they occupied in life. 

 This is a puzzle in animal mechanics too long to explain here, 

 but which a little thought will show is not so very difficult 

 to unravel, bearing in mind, as a clue, that the internal parts 

 are, as it were, inwards folds of the outside. 



