Fam. 3. Stenopodidae.—One or both legs of the third pair are longer and much stouter than those of the first two pairs. On a number of small anatomical points this family, including the littoral genus Stenopus from the Mediterranean and other warmer seas and Spongicola commensal with Hexactinellid sponges from Japan, is separated by some authors in a Tribe by itself.
Tribe 4. Caridea.
The third legs are not chelate. The third maxillipedes are 4–6 jointed, the end-joint of the second maxillipede nearly always lies as a strip along the end of the joint before it, and the first maxillipedes have a lobe on the base of the exopodites. The pleura of the second abdominal segment overlap those of the first. The abdomen has a sharp bend; the branchiae are phyllobranchs.
Fam. 1. Pasiphaeidae.—In this family the end-joint of the second maxillipedes is normally formed, and exopodites are usually present on all the thoracic limbs. Rostrum small or wanting. Rather numerous genera are known, most of which inhabit the deep sea, though a few come into the littoral zone. Pasiphaea chiefly in the deep sea, Leptochela in the tropical littoral zone.
Fam. 2. Acanthephyridae.—The end-joint of the second maxillipede is modified as in other Caridea, and the rostrum is very strong and serrate, but in the presence of exopodites, and in the form of the mouth-parts, this family agrees with the preceding. It is also a characteristic deep-sea family. Acanthephyra, Hymenodora, Nematocarcinus, etc.
Fam. 3. Atyidae.—This is an entirely fresh-water family, especially characteristic of the rivers and lakes of the tropics, some of the forms being exceedingly large and taking the place of the Crayfishes in these waters. Characteristic of this family is the fact that the fingers of the chelae are spoon-shaped, and carry peculiar tufts of bristles. Exopodites are present on the thoracic limbs of some of the genera (Troglocaris, Xiphocaris from Australia and the Malay Islands, Atyephyra from S. and W. Europe), but are absent in others. Caridina, widely spread and common in Indo-Malay and Africa; Atya from West Indies, West Africa, and Pacific Islands.
Fam. 4. Alpheidae.[[137]]—The exopodites are absent, and the rostrum is absent or very feeble. The chelae are powerful, and usually very asymmetrically developed. Alpheus has an enormous number of species which live chiefly in the tropical seas, where they haunt especially the coral-reefs, making their homes among the coral or in sponges, etc. Although occurring in the Mediterranean they penetrate very rarely into colder seas.
Fam. 5 Psalidopodidae.—This family, characterised by the absence of chelae on the second thoracic limbs, which carry instead a terminal brush of hairs, and by the rudimentary condition of the eyes, is represented by the genus Psalidopus from the deep waters of the Indian Ocean.
Fam. 6. Pandalidae.—The first thoracic limb is without chelae, only six-jointed. The rostrum is large and toothed. The genus Pandalus has numerous representatives in the northern littoral, P. annulicornis being one of the prawns most commonly met with in the fish-markets.
Fam. 7. Hippolytidae.—The first and second thoracic limbs bear chelae, the carpus of the second being divided into two or more segments. The first pair of chelae are not distinctly stronger than the second. Virbius has many species in the littoral zone of all seas, and one species, V. acuminatus, is pelagic. Hippolyte also has numerous littoral forms distributed all over the world, but chiefly in the arctic or subarctic seas. H. varians, common on the English coasts, shows interesting colour-reactions to its surroundings.[[138]]