Their various families are widely spread over the seas. It is well known that the barnacles frequently attach themselves in such vast numbers to ships' bottoms as materially to obstruct their way, and the acorn-shells often line the coasts for miles and miles with their large white scurfy patches. The Coronulæ settle so profusely on the skin of the Greenland whale as often to hide the colour of its skin, while the Tubicinellæ exclusively occur on the huge cetaceans of the South Sea. Some of the larger sea-acorns are highly esteemed as articles of food. The Chinese, after eating the animal of _Balanus tintinnabulum_ with salt and vinegar, use the shell, which is about two or three inches high and an inch in diameter, as a lamp, and the flesh of _Balanus psittacus_ on the southern parts of the South American coast is said to equal in richness and delicacy that of the crab.
While the Cirrhipeds grasp their prey as in a living net, the Siphonostomata lead a parasitic life chiefly upon fishes, sucking their juices with a bloodthirsty proboscis. Some (Argulus, Caligus) wander about freely on the body of their victims as grazing animals on their pasture grounds, or even make excursions in the water, where they will turn over and over several times in succession like mountebanks; others (Lerneæ), after having, like the barnacles, indulged in a vagabond existence in their first youth, remain ever after clinging to the spot on which they originally settled, and where their body undergoes such remarkable transformations that not a vestige of the crustacean structure which characterised their erratic life remains.
As we continue to proceed from the lower to the higher forms, we find, on the next stage of crustacean life, the numerous families of the Entomostraca; some bristly-footed (Lophyropoda), with a small number of legs and with respiratory organs attached to the parts in the neighbourhood of the mouth, others gill-footed (Branchiopoda), with numerous foliaceous legs, serving both for respiration and swimming. Some of these creatures, which are generally of such minute size as to be only just visible to the naked eye, have an unprotected body (Branchipus), but generally they are enclosed within a horny or shelly casing, which sometimes closely resembles a bivalve shell in shape and in the mode of junction of its parts, whilst in other instances it forms a kind of buckler, an opening being left behind, through which the members project.