METAMORPHOSES OF CRUSTACEA. 



243 



number of legs ; there are some which change their form com- 

 pletely during the first part of their life. This is the case, for 

 example, with the common Crab, 

 whose early condition is represented 

 in the accompanying figure. It is 

 remarkable, however, that in other 

 animals of the same Order, such as the 

 Cray-fish, the change of form is so in- 

 considerable, as not to deserve the name 

 of metamorphosis. Hence we perceive that 

 the degree of this change cannot be em- 

 ployed as a character in the subdivision 

 of the Class, as we have seen that it may 

 be in Insects. Of a large proportion of 

 Crustacea, however, the early form is 

 unknown. The metamorphoses of some 

 F, G . 470. EARLY FORM OF THE of the lower tribes are even more ex- 



CRAB. 



traordinary than those of the higher 

 (Figs. 496 and 497, 501 and 502) ; being frequently such, as 

 appear to remove the adult altogether from the class, to which 

 the larva evidently belongs. 



781. The early condition of many of the higher Crustacea 

 bears a strong resemblance to the permanent forms of the lower. 

 This is in no point more remarkable, than in the character of the 

 respiratory apparatus. Thus in the earliest period of the develop- 

 ment of the Astacus Jluviatilis, or River Cray-fish, no trace of 

 gills can be discovered ; but as the embryo within the egg 

 approaches maturity, temporary gills are developed in the form 

 of leaf-like expansions, occupying the situation of the extremities 

 of the maxillary appendages, which are the first developed of all 

 the members. These soon subdivide, and one part assumes a 

 cylindrical form, and seems no longer to belong to the apparatus ; 

 whilst branchial filaments begin to appear on the other which 

 are subsequently prolonged into complete gills. During this 

 interval, the thoracic extremities have made their appearance ; 

 and they also become furnished with branchial appendages. At 

 a subsequent time, a narrow groove or furrow is seen along the 



