The Stomapods are all swimmers; they have long bodies with a carapace; but it is so varied in form and size, that no general description of it can be given. They have external, instead of internal, organs of respiration; gills in the form of tufts are in some cases attached to a few of the foot-jaws, but they are much more frequently fixed to the basal joints of their swimming feet, so that the blood in their capillary veins is aërated through their thin skin as they float in the water. In the Squilla mantis, or S. Desmarestii, members of a genus of this family, the gills, which are fixed to the basal joint of their last pair of feet, consist of a long conical tube, on each side of which there are numerous parallel tubes, like the pipes of an organ, and each of these has a row of many long cylindrical filaments that drag in the water. The mouth and its appendages are similar to those of the common Decapods, with the exception of the anterior jaw-feet, which are of a singular and formidable structure. They are bent outwards, and their basal joint is exceedingly large, broad, and compressed; the next joint is less, with a groove in its side; the third joint is a blade like a scythe, whose cutting edge is furnished with long pointed teeth. The Squillæ are carnivorous, and, if any unfortunate animal comes within their grasp, they bend back the toothed edge of the first joint into the groove of the second joint like a clasp-knife, and cut it in two. These prehensile foot-jaws, or ‘pattes ravisseurs,’ are like the fore-feet of the praying Mantis, and like them weapons of defence. The genus Mysis, or Opossum Shrimps, have a long straight carapace, which covers most of the thorax, and folds down on each side so as to conceal the base of the feet: in front it is narrow, and ends in a flattened beak; at the posterior end it is deeply scooped out. The two last rings of the thorax are more or less exposed; the tail is long, almost cylindrical, tapering to the end, and terminating in a swimming fin composed of five plates spread like a fan. Both pairs of antennæ have jointed stems ending, the outer in one, the inner in two very long many-jointed filaments. On the top of the basal joint of the outer pair there is a very long lamellar appendage, ciliated on the side next the joint. Between the second and third joints of the exterior antennæ, Mr. Spence Bate found the organ of taste: the aperture is simply covered by a membrane, as in the lobster. The ears are in the last appendage of the tail.