Arthropoda include animals with a segmented, chitinous exoskeleton and with jointed appendages, variously modified for locomotion, feeding, and other activities.
Class Crustacea. Entomostraca. This group, formerly considered a subclass, is of convenience in designating a large assemblage of small, primitive crustacea belonging to several subclasses and orders distinguished from the higher crustacea, or Malacostraca.
Suborder Cladocera. Only a few occur in the sea. Examples: Podon, Evadne, sometimes important in neritic plankton. Very numerous in fresh water.
Order Ostracoda. This order includes more than 2000 species, mostly marine, living in the plankton and on the bottom (fig. 227b).
Order Cirripedia. These are the barnacles which as adults have calcareous shells and live sessilely in all benthic habitats, especially coastal. Some grow attached to drifting objects or upon whales and other animals, or they may form special floats for suspension. There are about 500 species, all marine.
Order Copepoda. Though small in size (about 0.3 mm to 8 mm in length), the copepods bulk large in the animal substance of the sea, for they are by far the most abundant of all crustaceans and usually constitute about 70 per cent of the zooplankton. There are over 6000 species of copedods, found mostly in the sea, where some 750 species are planktonic and extremely numerous. Many others are benthic or parasitic. The three main suborders of free-living forms are Calanoida (fig. 227c), Cyclopoida (fig. 229d), and Harpacticoida (fig. 229a). The first two are mainly pelagic,― 310 ―the last benthic. Like other Entomostraca and some Malacostraca, they gather food by means of fine bristles on certain appendages (p. 887).
subclass malacostraca. These are the large crustacea, mostly benthic, many with strong claws and biting mouth appendages.
Order Mysidacea. There are about 300 species, mostly marine, living on or near the bottom.
Order Cumacea. About 400 species of this order are known; nearly all are marine, benthic.
Order Euphausiacea. These are commonly known as “krill,” and in some regions are very abundant in the plankton and near or on the bottom. Some attain a length of about 50 mm, and may at times be the major constituent of the zooplankton. There are 85 known species, all marine. Examples: Euphausia, Meganyctiphanes (fig. 227a).
Order Amphipoda. There are about 3000 species, nearly all marine, in various habitats.
Order Isopoda. Over 3000 species are known; they are mostly marine, living on the bottom and on vegetation or burrowing in wood. Examples: Limnoria, Munnopsis (figs. 77 and 221).
Order Stomatopoda. This order contains about 200 species, all marine, benthic, most common in shallow water of lower latitudes.
Order Decapoda. Decapoda include crabs, lobsters, shrimps. They are widely distributed in both the pelagic and benthic regions. Most of the over 8000 species are marine.
Class Arachnoida. This class is well represented in the sea by a number of marine mites, over 400 species of sea spiders or pycnogonids, and 5 species of Limulus, the king crab. All are benthic.
Class Insecta. Only one insect is submarine during its whole life; a few others live on the foreshore or skip over the surface in search of food. Example: Halobates.