iv FAMILIES OF OPERCULATA 91 



(Fig. 02, HI) derived partly from the external hypodermis and 

 I >artly 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 

 pieces that go to make 

 up the skeleton; thus c - L y^ "\ 



starting with the typi- .f 

 cal number eight (Fig. ^. 

 63, A), we find that RL 



in various degrees a R 



A, 



fusion between neio-h- 



boiirill"" TjieceS ha.S ^ If;- ^- Diagrams of shells of Operculata. A, <'n/n- 



pJintgmus (Octomerklae) ; B, Balamis, Coroniiia, etc. 

 taken place in the (Hexamerklae) ; C. T>'f <<'</ ifn (Tetrameriihu-). C, 



different families. i ; lia ; C - L \ <-lateral : /-, lateral : R, rostrum ; 



Ji.L, rostra-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 SIM -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 

 lironght about liy 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 find unfused (Fig. ().">, A). The 

 majority of the species come from the Southern hemisphere, .//. 

 the members of the genera CV/o/'///v/// /////* and Or/tnin'/-/'*. hut 

 I'lirlii/hixiiKt. n'li/it ii/i'n in occurs in deep water in the Mediter- 

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



Fam. 3. Hexameridae. This family includes l>y tar the 

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

 are present, the laterals having fused \\illi the carmo -laterals 

 fFig. ()", B). The very large genus Holmm* belongs here, the 

 common I!. Unit it iminil u in of our coasts bein^ found all over the 

 world, and occurring under a number of inconstant varidaj forms. 

 Kspecial interest, all, aches to certain other genera, from their 

 habit of liviiii! parasitically on soft -bodied animals, whose llesh 

 they penetrate. 



('<>ronnl<i ilnnli'inii and Tnl'ici in'lln trachedlis live embedded 



