As the Crustacea pass parallel to the Conchozoa or Shell-animals, so do they divide into three cohorts or groups: the Mussel-, Snail-, and Kracken-like Crustacea, or Crabs, Wood-lice, and Spiders.
Cohort I. Mussel-like Crustacea—Crabs.
3508. Head and thorax connate, being mostly covered with a shell or shield, abdomen stunted or caudiform; maxillæ and branchiæ present.
Order 1. Lochmuschelartige, Pfriemenkrebse.
3509. The feet mostly simple and pointed, without large claws or forcipes, and furnished with setæ or lamelliform branchiæ; eyes sessile.
Fam. 1. Pholadoid, Mussel-Insects.
Small, almost microscopical, and slightly annulated animals with an uni-or bi-valvular dorsal shield or testa, stunted maxillæ, and few legs, having attached to them setiform branchiæ.
These animals are found in all stagnant waters, in which they are incessantly paddling or rowing themselves about. They remind us of those Infusoria which, like Brachionus, are covered by a scute or shield. They are what have been called Monoculi (Entomostraca or Lophyropoda), e. g. Daphniæ.
Number of feet small, and beyond their appended setæ there are no branchiæ; two eyes frequently blended together; antennæ mostly furcate like the feet; sexes separate; a perfect circulation within a true Mussel-like ventral cavity. They are microscopic Mussels with eyes and feet.
Fam. 2. Tellinoid, Branchiopoda.