CRAYFISH, LOBSTERS, CRABS, SHRIMPS, ETC. 115 



fish-lice which are especially interesting because of their 

 parasitic habits and the greatly modified structure resulting 

 therefrom. Some live as commensals, that is, are associated 

 with their hosts in such a way as to derive benefit from them, 

 without injuring them. Others are truly parasitic and live 

 upon the blood or tissues of their host. The most common of 

 these attach themselves to the gills of fishes, but they may also 

 be found as external or internal parasites, of whales, molluscs, 

 marine worms, starfishes and many other animals that are 

 found in the sea. 



The barnacles, order Cirripedia, look but little like other 

 Crustacea. For a long while they were classed with the 

 Molluscs, but a study of their 

 development showed that in the 

 young stages they are like the 

 crustaceans and not the molluscs. 

 The young are free-swimming, 

 and after going through a series 

 of changes they become attach- 

 ed to some firm object. Here 

 the young barnacle undergoes 

 further metamorphosis, and the 

 adult form with its compact or 

 somewhat worm-like body is de- 

 veloped. The appendages, or 

 legs, are long, slender and curled. 

 The animal is enclosed in a shell 

 often of the shape of a truncate 



FIG. 44. A stalked barnacle, 

 Lepas hillii. (About | natural 

 size.) 



cone and composed of six or more plates. The open end of 

 the shell may be closed by a lid or operculum, thus well pro- 

 tecting the inmate. The barnacle feeds on minute organisms 

 which it sweeps into its mouth by means of the long feathery 

 appendages. 



Although barnacles may completely cover piling and other 

 timbers in the water, they do not injure them and may even 

 be of some service in protecting them from wood-boring or 

 wood-destroying animals. Sometimes, however, they are 

 serious pests on oyster beds. The goose-barnacle, or ship- 



