The members of this order are the most highly organised of all the Crustacea, as well as being those which are most familiarly known, the Lobsters, Crabs, Shrimps, etc, being comprised under this head. For the most part they are aquatic in their habits, and they are usually protected by strong resisting shells. There is always a complicated set of "gnathites," or appendages modified for masticatory purposes, surrounding the mouth. The ambulatory feet are made up of five pairs of legs (hence the name of the order); the first pair - and often some other pairs behind this - being "chelate," or having their extremities developed into nipping-claws. The branchiae are pyramidal, and are contained in cavities at the side of the thorax. The carapace is large, covering the head and thorax, and the anterior part of the abdomen. The heart of the Decapoda is in the form of a more or less quadrate sac, furnished with three pairs of valvular openings. As regards the development of the Decapods enormous differences obtain, even amongst forms very closely allied to one another.

Fig. 156.   Squilla mantis, the Locust Shrimp.

Fig. 156. - Squilla mantis, the Locust Shrimp.

The Decapoda are divided into three tribes, termed respectively the Macrura, Anomura, and Brachyura, and characterised by the nature of the abdomen.

Tribe A. Macrura. - The "long-tailed" Decapods included in this tribe are distinguished by the possession of a well-developed abdomen, often longer than the cephalothorax, the posterior extremity of which forms a powerful natatory organ or caudal fin. As regards the development of the Macrura, most appear at first in the form of "Zoeae;" * but there is little metamorphosis in the common Lobster, and there is said to be none in the Cray-fish (Astacus fluviatilis). Fritz Muller, again, has shown that the primitive form of one of the Shrimps (Peneus) is that of a "Nauplius." Lastly, the young of the Spiny Lobster (Palinurus vulgaris) are transparent Phyllosomae, resembling Stomapods in appearance. This section comprises the Lobster, Cray-fish, Shrimp, Prawn, etc, of which the Lobster may be taken as the type.

In the Lobster (figs. 136, 137), as also in the Cray-fish (fig. 157), the somites of the head and thorax are amalgamated into a single mass, the "cephalothorax," covered by a carapace or shield, which is developed from "the lateral or epimeral elements of the fourth cephalic ring, which meet along the back, and give way preparatory to the moult. The tergal elements of the thoracic rings are not developed in either Crabs or Lobsters; when these rings are exposed by lifting up the cephalothoracic shield, the epimeral parts alone are seen, converging obliquely towards one another, but not joined at their apices " (Owen).

* The young Decapod, in most cases, leaves the egg in a larval form so different to the adult that it was originally described as a distinct animal under the name of Zoea. In this stage (fig. 160) the thoracic segments with the five pairs of legs proper to the adult are either wanting or are quite rudimentary. The abdomen and tail are without appendages, and the latter is composed of a single piece. The foot-jaws are in the form of natatory forked feet, and the mandible has no palp. Lastly, there are no branchiae, and respiration is carried on by the lateral parts of the carapace. The "Zoea" is separated from the "Nauplius" by having a segmented body, large paired eyes (sometimes with a median eye), and a carapace. The form proper to the adult is not attained until after several moults, constituting a genuine metamorphosis, though one which is effected by very gradual stages.

The first segment of the head bears the compound eyes, which are supported upon long and movable eye-stalks or peduncles. Behind these come two pairs of jointed tactile organs, the larger called the "great antennae," the smaller the "antennules." The mouth is situated on the under surface of the front of the head, and is provided from before backwards with an upper lip ("labrum"), two "mandibles," two pairs of "maxillae," three pairs of "maxillipedes" or "foot-jaws," and a bifid lower lip, or "metastoma" (fig. 158). The five remaining segments of the thorax carry the five pairs of ambulatory legs, of which the first constitute the great claws, or "chelae;" the next two pairs are also chelate, though much smaller; and the last two pairs are terminated by simply pointed extremities.

Fig. 157.   The common Cray fish [Astacmfluvi atilis), viewed from below. a Antennules; b Large antennae ; c Eyes ; d Opening of auditory sac ; e Last pair of foot jaws ; f One of the great chelae ; g Fifth thoracic limb ; h Swimmerets; i The last pair of swimmerets; j The opening of the anus below the telson.

Fig. 157. - The common Cray-fish [Astacmfluvi-atilis), viewed from below. a Antennules; b Large antennae ; c Eyes ; d Opening of auditory sac ; e Last pair of foot-jaws ; f One of the great chelae ; g Fifth thoracic limb ; h Swimmerets; i The last pair of swimmerets; j The opening of the anus below the telson.

Fig. 158.   Gnathites of the Cray fish (Astacus fluvia tilis). a Mandibles ; b Maxillae ; c Second pair of maxillae ; d First pair of foot jaws ; e Second pair of foot jaws ; f Third pair of foot jaws.

Fig. 158. - Gnathites of the Cray-fish (Astacus fluvia-tilis). a Mandibles ; b Maxillae ; c Second pair of maxillae ; d First pair of foot-jaws ; e Second pair of foot-jaws ; f Third pair of foot-jaws.

The segments of the abdomen carry each a pair of natatory-limbs, or "swimmerets," the last pair being greatly expanded, and constituting, with the "telson," a powerful caudal fin. Most posteriorly of all is the post-anal plate, or "telson," which may be looked upon either as an azygous appendage, or as a terminal segment which has no lateral appendages.