A BIOLOGY OF CRUSTACEA 



history is quite straightforward, but the hranchiurans differ from 

 copepods in that they continue moulting when adult. 



PARASITIC CIRRIPEDIA 



We have already seen (p. 102) how harnacles attach themselves to 

 other animals and enjoy free transport. It is hut a short step from 

 this to hecoming a true parasite. Anelasma sqaalicola seems to he 

 in the process of taking this step. The lower part of the body is 

 embedded in the skin of sharks, and gives off numerous root-like 

 processes. The free outer part of the body bears appendages, but 

 these are stumpy and lack the setae normally found on a barnacle. 

 The creature has a mouth and an anus, so it presumably obtains 

 some food from the water through which it is transported, but it 

 also seems likely that it obtains food from the tissues of its host, 

 via the rooting processes. 



Another barnacle, Rhizolepas annelidicola, roots itself into a 

 South African polychaete worm; it lacks a rnouth and an anus, so 

 that it must obtain all its food from the host. The part projecting 

 from the host is similar in many respects to a normal stalked 

 barnacle, but the mantle is reduced to a semi-cylindrical flap and 

 does not completely enclose the body. The appendages are quite 

 well developed, but there are no mouthparts. 



Anelasma and Rhizolepas seem to have evolved from stalked 

 barnacles by the simple development of rooting processes from the 

 stalk. This is very different from the development of the rooting 

 processes in the Rhizocephala. The name Rhizocephala means 

 1 root head ', and was given because it was thought that the rooting 

 system was derived from the head. In fact the origin is very differ- 

 ent, as the classical studies on the development of Sacculina have 

 shown. 



Sacculina is a genus with over eighty species, but one species, 

 5. carcini, has been studied in great detail. This species is mainly 

 a parasite on the common shore crab, Carcinus maenas, but it has 

 been recorded on eight other species of the same family. The adult 

 parasite appears as a sac on the under-surface of the crab, but its 

 roots extend all through the host's body, even to the tips of its legs. 

 Eggs, produced by the adults, give rise to nauplii which moult four 

 times then change into cypris larvae in the usual barnacle fashion. 

 The cypris finds a young crab, and attaches itself, by means of its 

 antennules, to a seta, which may be located anywhere on the body 



