Krill (Euphausiids), are small and exclusively marine crustaceans of the order Euphausiacea, found in all the world's oceans. The name "krill" comes from the Norwegian word krill, meaning "small fry of fish", which is also often attributed to species of fish.
Krill
Krill anatomy explained, using Euphausia superba as a model
The gills of krill are externally visible
A nauplius of Euphausia pacifica hatching, emerging backwards from the egg
Crustaceans are a group of arthropods that are a part of the subphylum Crustacea, a large, diverse group of mainly aquatic arthropods including decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the hexapods emerged deep in the Crustacean group, with the completed group referred to as Pancrustacea. The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans.
Image: Sally Lightfoot crab (4202519454)
Image: Humarus gammarus 1
Image: Procambarus clarkii
Image: Lysmata amboinensis in Tropicarium Oceanarium Budapest