The suborder NATANTIA includes the numerous species of what are commonly known as Prawns and Shrimps. These are characteristically powerful swimmers, with lightly armoured bodies, more or less flattened from side to side, with a thin, saw-edged rostrum, and with large swimmerets which are the chief organs of swimming; in addition, some of the more primitive Natantia have swimming branches, or exopodites, like those of the Euphausiacea, on the thoracic legs. This suborder is divided into three tribes. The _Penæidea_ include the large Prawns of tropical seas (_Penæus_--Plate IV.), which have the first three pairs of legs provided with chelæ, and not differing greatly in size. The _Stenopidea_ are a small group of forms resembling the _Penæidea_ in having chelæ on the first three pairs of legs, but the third pair are much larger than the others. The _Caridea_ comprise our common Prawns (_Leander, Pandalus_) and Shrimps (_Crangon_), besides a host of less generally known forms; in these the third legs are never chelate, although the first and second often are. [Illustration: _PLATE IV_ _Penæus caramote_ FROM THE MEDITERRANEAN. (ABOUT HALF NATURAL SIZE) (_From Brit. Mus. Guide_)] The second suborder, that of the REPTANTIA, is much more diversified, but the animals composing it are united by certain characteristics, of which the most obvious are their creeping habits (although some species can swim well), their heavily armoured bodies, often more or less flattened from above downwards, with the rostrum never thin and saw-edged, and the swimmerets not used to any great extent for swimming. The first section of the Reptantia, the _Palinura_, includes the Spiny Lobsters, Rock Lobsters, or Sea-Crawfish, and their allies, forming the tribe _Scyllaridea_. They are distinguished by having no large pincer-claws, though the last pair of legs may have small pincers in the female sex. One species, the Common Spiny Lobster (Plate V.), is found on the southern and western coasts of the British Islands. The other tribe belonging to this section is the _Eryonidea_, comprising a number of small lobster-like forms living in the deep sea. They have pincer-claws on the first four, or on all five, pairs of legs, and they are of special interest on account of their geological antiquity. [Illustration: _PLATE V_ THE COMMON SPINY LOBSTER (_Palinurus vulgaris_). (MUCH REDUCED) (_From Brit. Mus. Guide_)] The section _Astacura_ contains only a single tribe, _Nephropsidea_, formed by the true Lobsters and the fresh-water Crayfishes. They have pincer-claws on the first three pairs of legs, and the first pair are much larger than the others.