60 PRIMITIVE ANIMALS 



not regard them as animals at all but as a tumour 

 produced by the crab itself, and successive naturalists 

 regarded them as lowly organized worms or molluscs. 

 The adult structure consists of a bag attached beneath 

 the crab's tail, in which the reproductive organs are 

 situated and at maturity large quantities of eggs are 

 produced; this bag communicates, at the point of 

 attachment, with the interior of the host by means of 

 a ramifying system of roots which penetrate to all 

 parts of the crab's body and absorb nourishment 

 from its blood. There is nothing in this structure to 

 suggest a Crustacean or indeed any particular group 

 of animals. But the eggs of the parasite hatch out 

 as little free-swimming larvae possessing all the 

 characteristics of the typical Crustacean larva, the 

 Nauplius (Fig. 1 1 A), with its three pairs of append- 

 ages, and not only is the Nauplius larva of the 

 Rhizocephala a typical Crustacean Nauplius, but it 

 shows certain structures, e.g. the lateral horns, typical 

 of the Cirripede Nauplius, the larval form assumed by 

 all Barnacles before they become attached to rocks, 

 keels of ships, etc. 



Now the Nauplius of the Rhizocephala, after 

 swimming about in the sea for four days, undergoes 

 a moult and becomes metamorphosed into a totally 

 different larval form, the Cypris (Fig. HB), with a 

 single pair of antennae and six biramous thoracic 

 limbs. This Cypris larva is absolutely characteristic 



