THE EVIDENCE FROM DEVELOPMENT. 



205 



this curious history comes to an end. Barnacle-growth therefore 



exhibits as its stages, firstly, a free-swimming larva or " Nauplius '* 



(Fig. 119), with its three pairs of legs 



or appendages ; then a pupa with its 



bivalve shell, its large feelers, its two 



eyes, and its six pairs of swimming feet 



(Fig. 120, B); and finally the eyeless, 



stalked, and degraded adult stage, in 



which, to quote the words of authority, 



a barnacle appears as a crustacean, 



"fixed by its head, and kicking the 



food into its mouth with its legs." 



From the crustacean array, we may 

 next select an animal which, whilst it 

 resembles the Barnacle in many of its 

 features, and especially in development, 

 is yet sufficiently distinct to lead towards 

 forms presenting greater differences in 

 the adult stage and yet exhibiting close 

 identity in the early phases of existence. Such a form is the 

 Sacculina (Fig. 118), a type of Crustaceans of the very lowest 

 grade, which live an' attached, rooted, and parasitic existence on 



FIG. 119. YOUNG OF BARNACLE. 



FIG. 120. DEVELOPMENT OF BARNACLES, &c. 



fishes or on other crustaceans. If a barnacle exhibits " retrograde 

 development " or physiological backsliding, in that it appears to be a 

 lower and more modified form when adult than when in the pupa- 

 stage, the Sacculina and its neighbours exhibit a still more degraded 

 condition. The organism just named, exists as a sausage-like bag 

 attached to the bodies of hermit-crabs. There exist no traces of a 

 mouth or, as Fritz Miiller remarks, " they lose all their limbs 



