52 KNOWLEDGE OF AFFINITIES DERIVED FROM METAMORPHOSIS. 



the name of Water-flea, that their real relationship is apparent. 

 In fact, they may be said to have been originally formed on 

 the same plan or type ; but to have afterwards departed from it, 

 each in a different direction, so as to become very unlike one 

 another in their adult forms. A still more striking illustration 

 may be derived from the metamorphosis of the animals of the 

 class of Cirrkopods, or Barnacle tribe (Figs. 11 and 12). These 

 were formerly placed among the Mollusca, on account of their 

 resemblance to that group, in having their soft bodies enclosed 

 within shells, in remaining constantly attached to some other 

 object, whether fixed or floating, and in the absence of any 

 distinct division of the body into segments. But it was pointed 

 out by Cuvier, that the arrangement of the arms and. of the 

 nervous centres is such, as to entitle them to a place in the 

 Articulated, rather than in the Molluscous series, that is, to 



be associated rather with 

 Insects, Spiders, and Crabs, 

 than with Oysters, Snails,- 

 and "Whelks. More recent- 

 ly, it has been shown that 

 they undergo a very curious 

 metamorphosis, having in 

 their early state a form 

 not very unlike that of the 

 early state of the common 

 Crab (Fig. 6), possessing 

 eyes, and the power of free 

 movement but afterwards 

 becoming fixed to one spot 

 for the remainder of their lives, losing their eyes, and forming 

 a shell, which, though composed of several pieces, has nothing 

 in common with the jointed shell of the Crab or Lobster. Ih-nee 

 the affinity of the Cirrhopods to the Crustacea, unlikely as it 

 seemed at first, has been clearly demonstrated by these obser- 

 vations. 



33. Now, we shall suppose that those difficulties !:ad l>'^n 



Fio. 11 SHELL o 



THB BARXACLK. 



FIG 1-2 HODY OF 

 THB BARXACLE. 



