ARTHROPODA — CRUSTACEA 305 



Sub-class 5, Cirripedia (Barnacles) 



Body imperfectly segmented ; posterior part of trunk vestig- 

 ial. The carapace of other Crustacea here forms a fleshy 

 mantle, usually strengthened by calcareous plates. 



These degenerate animals are marine, and fixed during adult 

 life, or they are parasitic. The fixed forms have the body and 

 limbs completely inclosed in a mantle strengthened by cal- 

 careous plates. They are attached by the anterior part of the 

 head either by a flattened disk (as in the acorn barnacle, — 

 Balanus), or by an elongate, muscular stalk (as in the goose or 

 ship barnacle, — Lepas) . 



Barnacles are a very interesting example of degeneracy, their 

 life history passing from a free-moving youth to an attached 

 maturity, accompanied by a degeneration of the body until 

 the whole posterior portion becomes vestigial. The young larva 

 is free-swimming, hatched as a free nauplius ; after several 

 molts this passes into the Cypris stage, with a bivalve shell like 

 an ostracod but with limbs and internal anatomy unlike this 

 sub-class. Later it attaches itself to some object by its anten- 

 nules aided by a secretion poured out from a gland at their base ; 

 soon the parts of the new, changed shell appear beneath the old 

 one, which is molted, and the adult form is thus attained. 



The Cirripedia are known from the Ordovician to the present, 

 but have been abundant only since the upper Tertiary. 



Derivation of name. — Latin cirrus, a fringe, + pes, foot ; 

 referring to the fringe-like feet which the animal can protrude 

 from its shell and by means of which it respires and gathers food. 



Examples are Balanus (Eocene to present ; attached to rocks 

 between tide levels), Lepas (Pliocene to present; growing on 

 the bottom of ships, etc.), Tubicinella (living deeply embedded 

 in the skin of whales), Anelasma (living embedded in the skin 

 of sharks and partially parasitic by means of roots), Sacculina 

 (living entirely parasitic on crabs, its roots penetrating all parts 

 of the body except heart and gills). Probably the best known of 



