20 NATURAL HISTORY OF ARTHROPODS. 



one of the siphonostomous Crustacea (to be referred to further on), which in turn was 

 parasitic upon the large sunfish, Mola rotundata. Lepas, the typical genus of this 

 family, is almost invariably attached to floating objects, and, hanging down in the 

 water, sometimes reaches a length (in L. anatifera, Fig. 26) of sixteen inches. These 

 are the most common forms in collections, and the species have a world-wide distri- 

 bution. 



FIG. 26. Lepas anatifera, attached to floating pumice-stone. 



FAMILY VEBRUCID^E. -This family, containing but the single genus Verruca, 

 closely resembles the next in being sessile, but differs in having a very symmetrical 

 shell, and the valves closely resemble those of the Lepadidas. 



FAMILY BALANID^E. The acoi-n barnacles are the most numerous in species and 

 specimens, and we may take the common form of the Northern Seas, Balanus balan- 

 oides, as our type. This species, which is found encrusting rocks and piles between 

 tide-marks, in form is a white calcareous pyramid, made of six immovably-united 

 pieces, forming an irregular oval ring with the centre filled with four movable valves, 

 between which the animal protrudes the cirri. Inside of the shell we find the animal 

 closely resembling that part of a Lepas which is embraced within the capitulum, but 

 with an additional muscle for pulling down the occludent valves and thus more com- 

 pletely protecting it. Bdlanus psittacus, is the largest species, sometimes reaching a 



