Iv FAMILIES OF OPERCULATA 91 
(Fig. 62, H1) 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 be classified accord- 
ing to the number of 
C Cc 
pieces that go to make as aa CL+L eae 
up the skeleton; thus any e >) G 7 
starting with the typi- | { ) ) 
cal number eight (Fie. 1 'e) Re yy 
63, A), we find that ae ey Ms 
in various degrees a R R R.L+R+R.L 
fusion between neigh- 
. . ‘ } " qe m q F -eHlats lato 
ae anineee 
taken place in the (Hexameridae); C, Tetraclita (Tetrameridae). C, 
different families. CN ee ea ; ZL, lateral ; R, rostrum ; 
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 1s 
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 Catophragmus and Octomeris, but 
Pachylasma 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. tintinnabulum of our coasts being found all over the 
world, and oceurring under a number of inconstant varietal 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 Tubicinella trachealis live embedded 
