IV FAMILIES OF OPERCULATA 9 1 



(Fig. 62, HI) derived partly from the external hypodermis and 

 partly from the lining of the mantle. The hard parts of the 

 shell usually also contain spaces and canals (C). 



The various forms of Acorn-barnacle may he classified aecord- 

 iug to the number of 



pieces that go to make ^ — ^ ^^-^-^^ ^Y^ 



up the skeleton ; thus 

 starting with the typi- 

 cal number eight (Fig. 

 63, A), we find that 

 in various degrees a 

 fusion between neigh- 



bourintJ" nieces has ^^*^'' ^^' — DiagTams of shells of Operciilata. A, Cato- 



'-' phragmus (Octomeridae) ; B, Bulanus, Coronula, etc. 



taken place in the (He.xameriflae) ; C, Tefraclita (Tetrameridae). V, 



different families. " '^^^T^ ' P'^l f ""O"''-^*"-'^' ' ''' ^^'^"'"^ '• ^'' ™-^tn.m ; 



K.L, rostro-lateral. 



Fam. 1. Verru- 

 cidae. — The ancient genus Verruca, which is still widely dis- 

 tributed in all seas, and is found fixed upon foreign objects on 

 the sea-bottom at various depths, is interesting on account of 

 the asymmetry of its shell, which bears a different aspect accord- 

 iuij; to which side one regards it from. This asymmetry is 

 brought about by the skeletal pieces (carina, rostrum, and paired 

 terga and scuta) shifting their positions after fixation has taken 

 place. 



Fam. 2. Octomeridae. — In this family tlie eight plates com- 

 posing the shell are separate and unfused (Fig. 63, A). The 

 nuijority of the species come from the Southern hemisphere, e.g. 

 the members of the genera Oatophragmus and Octomeris, but 

 Pachylasina giganteum occurs in deep water in the Mediter- 

 ranean, where it has been found fixed upon Millepore corals. 



Fam. 3. Hexameridae. — This family includes by far the 

 greater number of the Acorn-barnacles, in which only six plates 

 are present, the laterals having fused with the carino-laterals 

 (Fig. 63, B). The very large genus Balanus belongs here, the 

 common B. tintinnahtdum of our coasts being found all over the 

 world, and occurring under a number of inconstant varieta.1 forms. 

 Especial interest attaches to certain other genera, from their 

 habit of living parasitically on soft-bodied animals, whose flesh 

 they penetrate. 



Coronula diadema and Tuhicinella trachealis live embedded 



