CHAPTER V

THE SUB-CLASS MALACOSTRACA

THE head and trunk are together composed of thirteen, or, if an ophthalmic ring be included, of fourteen segments. The caudal part or pleon is composed of six segments and a telson. The trunk is clearly distinguished from the pleon, but some part of it is always more or less closely united with the head. To every segment normally belongs a pair of jointed appendages. The eyes are either pedunculate, and limited to two in number, with rarely a pair of accessory ocelli, or they are sessile, and then generally two, but sometimes four, or with the components variously distributed. There are two pairs of antennæ, a pair of mandibles, and two pairs of maxillæ. Of the next eight pairs of appendages, from one to three are maxillipeds, organs of the mouth, the remainder, from seven to five in number, being prehensile or locomotive. All these are typically seven-jointed. Like the second antennæ and second maxillæ they may either have or be without an exopod on the second joint, and they may also have or be without an epipod on the first. The six pairs of appendages of the pleon, when present, generally have an exopod. The last pair almost always differs in character from the rest. The paired appendages of the mouth work from the sides, the oral aperture itself being fringed by the labrum or upper lip above, and the bifid labium or lower lip below. A short œsophagus leads up into the stomach. The intestinal tube terminates in the under side of the telson. The heart which is dorsally placed has lateral openings for the entrance of the blood that has been oxygenated in the branchiæ. These slits are in one, two,

or three pairs, only in the Squillidæ exceeding that number. The ganglia of the same pair are situated close to one another, though the commissures may stand a little apart. By the dorsal and lateral extension backwards and generally also forwards of one (or two) of the cephalic segments a shield or carapace is formed covering at least some part of the trunk and sometimes all of it.

The above characters will suffice for a descriptive definition of the Malacostraca, but it may be proper to remind the reader that the segments are sometimes so intimately coalesced that their separate identity is entirely obscured, and that moreover almost any pair of the appendages, even one so seemingly indispensable as the mandibles, may in certain cases be missing. Absence of eyes is by no means infrequent, and the telson, though perhaps never properly speaking absent, is often, by its close union with the preceding segment, so withdrawn from recognition, that in practice it is spoken of as absent.

Order 1.-Podophthalma.

In this order there is normally a pair of compound eyes on movable stalks, the eyes being sometimes absent but never sessile; the dorsal shield or carapace extends back over the ninth segment or further.

Sub-order 1.-Brachyura.

The carapace extends over the whole head and trunk, with occasional exception of the trunk's ultimate and penultimate segments, and is longer than the pleon. In the carapace are excavated orbits and fossettes, hollows respectively adapted to receive the stalked eyes and the short first antennæ. The third maxillipeds have some of the joints broad and flat, and they form a more or less complete operculum to the well-defined mouth cavity. The following pair of appendages are perfectly chelate limbs, commonly called the chelipeds. The next four pairs are adapted for walking or swimming, or rarely may have a prehensile character. In the sternal plastron, or breast

plate, the coalesced ventral plates of the last five segments of the trunk are distinguishable, and three earlier segments are obscurely represented. It is never entirely linear. The vulva of the females are generally placed upon it, but in some groups are transferred to the basal joints of the ante-penultimate legs. The pleon is of subordinate size, usually reflexed against the concavity of the plastron, in the male generally narrow and pointed, with only one or two pairs of pleopods, in the female broad, with four pairs of pleopods. The basal joint of the first antennæ contains auditory hairs but no otoliths.

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In this definition the Anomura apterura are included. To the dry bones of definition must be added an even less appetising explanation of terms in common use for the description of genera and species. The orbital regions of the carapace speak for themselves as being those which contain the eyes. The 'front' lies between them. Behind it on the under surface are the fossettes of the first antennæ, followed in the median line by the epistome, the buccal or oral frame, and the sternal plastron. The second antennæ are placed outside of and a little behind the first. The hind margin' of the carapace separates the trunk from the pleon, and lies between the first joints of the last pair of trunk-feet. Between it and the orbits are the lateral margins, each of which is subdivided into an antero-lateral and a postero-lateral portion forming, when not continuous, the epibranchial angle. The dorsal surface of the carapace is marked by several grooves corresponding with the insertions of muscles underneath, and also forming the boundary lines of regions which roughly coincide with the positions of important internal organs. Along the centre lie the gastric, cardiac, and intestinal regions, respectively over the stomach, heart, and intestine. The hepatic regions over the liver flank the gastric region on either side in front, and behind these lie the two branchial regions, the 'cervical groove' being that which separates the gastric and hepatic regions from the cardiac and branchial. On the under side the pterygostomian regions, 'the wings of the mouth,' lie between the antero

MASKS AND FACES

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lateral margins of the carapace and the buccal frame. Milne-Edwards remarks that the grooves are often emphasised about the middle of the carapace, so as to produce the appearance of the capital letter H, the transverse

FIG. 2.-Ethusa mascarone (Herbst). [Herbst]

line being the upper boundary of the cardiac region. In some cases the grooves are so arranged as to represent very strikingly a human countenance or the caricature of one, as in the Masked Crab of Great Britain and the grimacing Ethusa mascarone of the Mediterranean, which is here shown as depicted by Herbst. Such likenesses the old writers were not at all disinclined to accentuate.

The Brachyura are divided into tribes, in regard to which, however, there is not at present any absolute agreement among naturalists. We shall here arrange them under the names Cyclometopa, Catometopa, Oxyrrhyncha, Oxystomata, Anomala. It is melancholy, but scarcely avoidable, that an alternative list of names should have to be mentioned, for these tribes in the same succession may be called--Cancroidea, Ocypodiidea, Maioidea, Leucosiidea, and Anomura apterura. The subjoined table will be useful for reference.