PARASITIC CRUSTACEA 117 



of the crab. The larval limbs are cast off, leaving a small sac-like 

 creature attached to the base of the seta. A hollow dart-like structure 

 is developed; this penetrates the thin cuticle at the base of the 

 crab's seta, and the contents of the sac are injected into the body 

 of the crab. The small mass of undifferentiated cells migrates to 

 a position below the crab's intestine and then starts putting out roots 

 from a small central body. The roots spread throughout the host, 

 and the small central body enlarges, eventually making a hole 

 through the external skeleton of the crab and appearing on the 

 outside in the adult form. The complete cycle takes about nine 

 months. 



A crab which is infected and carrying an adult Sacculina cannot 

 moult, but nevertheless can continue living for at least a couple of 

 years. The reproductive organs of the crab are most seriously 

 affected by the parasite. The testes and ovaries are rendered func- 

 tionless, and the host usually dies without producing any offspring. 

 Infected male crabs often lack their secondary sexual characteristics. 

 This is particularly noticeable in the abdomen, which is usually 

 narrow in the males and broad in the females; infected male crabs 

 develop broad abdomens, and, in some species even develop the 

 egg-carrying appendages characteristic of the females. All the 

 changes observed in infected crabs have been towards feminisation. 

 The females do not change their external appearance when infected. 



Not all the Rhizocephala prevent their hosts from moulting. 

 Thompsonia, which infects alpheid shrimps, produces numerous 

 reproductive bodies all over the surface of its host, but these are 

 cast off after they have produced a single brood of eggs, and the 

 host can moult unhampered. Thompsonia is remarkable because a 

 single rooting system gives rise to numerous external reproductive 

 individuals, and successive generations of these are produced. The 

 reproductive bodies have only got ovaries, and males are not known, 

 so that parthenogenesis appears to be the rule. The whole structure 

 and way of life of this parasite resembles that of a fungus; only 

 the cypris larva gives a clue to its true identity. 



Yet another group of the cirripedes, the Ascothoracica, have 

 adopted a parasitic way of life, but their modifications are quite 

 different from those of the Rhizocephala. The essential feature of 

 the evolution of ascothoracicans has been progressive modification 

 from a type similar to a cypris larva. 



In Laura, which is a parasite on certain relatives of sea-anemones, 

 the two valves of the carapace have become enormous relative to 



