76 A BIOLOGY OF CRUSTACEA 



settle in a place already inhabited by others of its own species. 

 If a cypris cannot find such a place It can delay the change into 

 adult form for at least two weeks. The aggregation of barnacles in 

 this way is an obvious aid to reproduction; if barnacles lived in 

 isolation the chances of the eggs becoming fertilised would be very 

 small, for although most barnacles are hermaphrodite they mostly 

 cross fertilise. Because of their immobile adult state the search for 

 possible reproductive partners has shifted to the larval stage. 



The position taken up by a cypris in relation to water currents is 

 also important. The feeding mechanism is most effective when the 

 legs beat into a current; the cypris should therefore orientate so 

 that this will be possible when the adult form is assumed. In fact 

 it is found that the cypris tends to align itself with respect to 

 light, and this factor overrides all others to such an extent that a 

 cypris sometimes settles in a position which is disadvantageous with 

 respect to a water current. This can be corrected to some extent by 

 the adult barnacle twisting during its growth. In situations where 

 barnacles are exposed to a current from one direction, for instance 

 on a boat moored at one end, the adults become orientated in such 

 a way that the feeding limbs beat into the current. 



The life history of a malacostracan may be long and complicated, 

 or the creature emerging from the tgg may resemble the parents so 

 closely that all it has to do is to grow and become mature. This 

 direct type of development is found throughout the Peracarida, 

 Syncarida, Leptostraca and some of the Decapoda, particularly 

 those crayfish and crabs which live in fresh water. 



A full sequence of larval forms is found in the Euphausiacea; a 

 nauplius emerges from the egg, changes to a metanauplius, and this 

 develops a carapace and becomes longer in the trunk to form a 

 calyptopsis larva. This has paired eyes, but they are not freely 

 movable. When the eyes become stalked and movable, and the 

 pleopods have developed, the larva is called a furcilia. Later, as the 

 adult form is more closely approached the larva is called a cyrtopia. 

 The names of these larval forms were given at a time when they 

 were thought to belong to distinct genera, and before it was realised 

 that they all formed part of the developmental series of one type 

 of crustacean. 



The various stages of the Euphausiacea have their counterparts 

 in the development of the penaeid prawns. These also hatch as a 

 nauplius, change to a metanauplius, then go through stages called 

 protozoea, zoea and a post-larval stage which is very similar to 



