THE CLASS CRUSTACEA 317 



Subclass CiRRiPEDiA. 



This subclass, which is confined to the sea and its brackish inlets, 

 includes the barnacles and a number of degenerate, unsegmented, 

 often limbless and distorted forms which are either rooted or burrow- 

 ing parasites of invertebrate marine animals of many kinds. Though 

 fixed as adults, almost all of them begin life as an active nauplius, 

 and pass through a second free-swimming phase (Cypris stage), in 

 which they are enclosed in a bivalve shell and somewhat resemble 

 an Ostracod. From this transitory condition of activity the retrograde 

 change into the sedentary or parasitic adult is sometimes quite extra- 

 ordinary. Most of the Cirripeds are hermaphrodite, but in some of 

 the parasitic forms the sexes are separate ; in both cases dwarf 

 males, parasitic on the hermaphrodite or on the female, are to be 

 found. 



The barnacles, which encrust most things, living or inanimate, that 

 float in the sea or are washed by the waves, attach themselves by 

 their head, or rather by their antennules, where lie the openings of 

 special cement-glands for this purpose. The region of attachment is 

 sometimes produced to form a long fleshy stalk. The animal itself 

 is enclosed in a very complete fold of the integument, or mantle, the 

 outer surface of which is fortified by plates of shell. In the mantle- 

 cavity — loose, except for the fixation of the much-transformed head — 

 lies the body of the animal, with six pairs of biramous tendril-like 

 thoracic appendages vibrating at the opening of the shell and raking-in 

 food. In addition to these curly appendages, or cirri, there are three 

 pairs of short mouth-appendages ; but the antennules are altered and 

 functionless, and the antennse disappear altogether. 



Among the parasitic Cirripeds the order of Rhizocephala may be 

 instanced. These, which infest higher Crustacea, particularly crabs, 

 are, in the adult stage mere swollen bags, without segmentation, without 

 appendages, "sans everything" except ovary and testes. In the 

 absence of an alimentary canal nourishment is absorbed by a mass of 

 rootlets that ramify throughout almost every organ and tissue of the 

 "host'' in which they are embedded. A common form is Sacculina, 

 parasitic on, or in, crabs. Sacculina is hatched as a nauplius, and 

 becomes a " cypris," differing from those of other Cirripeds only in the 

 absence of an alimentary canal. After a brief existence in the free 

 state, the cypris larva first attaches itself to, and then after certain trans- 

 formations burrows into the crab, and, as an internal parasite, undergoes 

 a further remarkable series of transformations, or reconstructions, which 

 end in the formation of the ramifying absorptive roots and the sack-like 

 body of the adult. When these are fully grown the sack-like body 

 breaks through the soft integument of the crab's abdomen and becomes 

 external, the roots of attachment and absorption remaining deeply 

 embedded in the tissues of the host. 



