Crustacea has both direct and indirect development.
| Nauplius e.g. Cyclops | Characteristic of class Nauplius. Simplest and commonest, found in marine crustaceans and malacostracans. When development proceeds through many larval forms, Nauplius is the earliest and most basic larva. Single median eye. 3 pairs of appendages. Antennules: balancing. Antennae: Locomotor, food collection. Mandibles: Food collection. In Brachiopods, it develops straight into adults, but in other crustaceans it may go through intermediate larva forms. |
| Metanauplius e.g. Apus | Moulting and growth of Nauplius. Body is divided into broad cephalothorax and elongated abdomen. Terminates in a pair of caudal forks. 3 pairs of appendages as in Nauplius. Rudiments of 2 pairs each of maxilla and maxillipeds. Decapods, Stomatopods and Notostracans (e.g. Apus) |
| Protozoaea e.g. Euphausia | Marine prawns: Nauplius changes to Protozoaea. Broad segmented cephalothorax covered with a small carapace. Slender, unsegmented abdomen, bear no appendages, end in a forked telson. Single median Nauplius eye. Antennules, Antennae, Mandible, 1st and 2nd maxillipeds. |
| Zoaea e.g True crabs | In most marine decapods (except Peneids and Sergestids), hatching takes place at Zoaea stage. Broad cephalothorax and curved abdomen with a forked telson. Helmet like carapace with 2 long spines: median dorsal and median rostrum. Pair of large compound movable eyes. Maxillipeds are used for swimming. Rudiments of thoracic appendages. |
| Cypris e.g. Lepas | In Cirripedia, Nauplius changes into Cypris. Body and appendages are closed within a bi-valved shell provided with adductor muscle. Modified antennules have cement glands at their bases. 6 pairs of thoracic limbs. |
| Mysis e.g. Penaeus | In Peneids, Zoaea is modified into Mysis. 13 pairs of appendages. In prawn, it marks the end of life history. But in some, Nauplius and Zoaea are passed through in the egg, so Mysis represents the first life history. |
| Megalopa e.g. Crabs | In true crabs, Zoaea changes into Megalopa. Resembles adult crab and has 13 pairs of appendages. Abdomen has 6 pairs of Pleopods. |
| Phyllosoma e.g. Palinurus | Modified Mysis stage. Body is distinguished into head, a transparent thorax and abdomen. Compound, stalked eye. |
| Alima | Modified form of Zoaea found in some Malacostracan which hatches from egg. Short and broad carapace. 6 segmented abdomen with 4-5 pairs of Pleopods. Well formed second maxillipeds and armature of telson. |
Significance of Larval forms
In line with Biogenetic law (Successive stages of individual development corresponds to ancestral forms), Nauplius can be considered similar to the ancestral stock from which present day crustaceans have evolved. Zoaea, Megalopa etc. represent stages of evolution.
Finding out homologies and ancestry among different groups.
Helpful in wide distribution of species.
Because of a active larval stage, food reserves of eggs can be kept to a minimum.