FAMILIES OF OPERCULATA 



91 



(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 forma of Acorn-barnacle may be classified accord- 

 ing to the number of 

 pieces that go to make 

 up the skeleton ; thus 

 starting with the typi- 

 cal number eight (Fig. 

 63, A), we find that 

 in various degrees a 

 fusion between neigh- 



bourinc pieces has ^^'^''^ ^'^' — Diagrams of sliells of Operculata. A, C'ato- 



, , . phragmus (Octomerklae) ; B, Balamis, Coronula, etc. 



taken place m the (Hexameriilae) ; C, Tefmdita (Tetrameridae). 0, 



different families carina ; O.L, carino-lateral ; L, lateral ; R, rostriim ; 



Ji.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- 

 ing 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 the eight plates com- 

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

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

 the members of the genera Gatophragmns and Octomeris, but 

 Pachylasma giganteutn 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. tintinnctlndum of our coasts being found all over the 

 world, and occurring under a number of inconstant varietaj 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 



