206 



LIFE BY THE SEASHORE. 



moults and becomes converted into the megalopa, a form 

 much more like a crab than the zoea, but differing markedly 

 from crabs in the presence of a long, mobile abdomen, 

 capable of being used in locomotion. The megalopa is the 

 stage of transition from the free-swimming zoea, whose 

 habitat is the open sea, to the creeping crab, whose habitat 

 is the sea-floor. Its special interest lies in the fact that 

 while the zoea swims by means of its thoracic appendages, 

 as do some of the' lower Crustacea, the megalopa can swim 

 with its tail like a long-tailed Decapod. For a full dis- 

 cussion of development of the Crustacea 

 reference must be made to the text- 

 books, but the study of a living 

 megalopa will give you a more real and 

 vivid appreciation of the process than 

 the clearest and best description. The 

 megalopa stage of our common crabs 

 may often be found among weeds in 

 the rock pools. 



When found, place your specimens 

 in a saucer of clean water, and examine 

 of tail and body, the w ith a lens. If you have obtained 



ten legs, the rostrum . . ,.~, ^ , .,, 



between the eyes, and specimens of different ages you will 

 tail SV After e B e ro S ok n the no ^ ce now some move like a Galathea 

 by rapid jerks of the tail, how others 

 alternate between this and creeping like a crab, while others, 

 again, confine themselves almost entirely to the latter form 

 of motion. The sight is one which you will probably mark 

 as forming an epoch in your observations of shore animals. 

 The fact in itself is a 

 mere trifle perhaps, but 

 it is one of those ap- 

 parently trifling pieces of 

 observation which seem 

 to suddenly illumine 

 days of patient, but ap- 

 parently fruitless, study. 

 Later the little mega- 

 lopa tucks in its tail, FlQ 59._My 8 is stage of Norway lobster (Neph- 

 UnderCOeS Certain minor rops). Notice that the biramose legs of the 

 . , i larva are in process of transition into the 



alterations, and becomes uniramose legs of the adult. After Sars. 



