A HISTORY

OF

RECENT CRUSTACEA

THE MALACOSTRACA

CHAPTER I

OUTLINE OF CLASSIFICATION

It is conceivable that by origin all the animals of the globe belong to a single family. They now exhibit very great divergence. Between a star-fish and a crocodile, for example, the cousinship is obscure and remote. Yet almost all species may be included within a few principal clans, and these are united one to another by a small number of intermediate forms of life. For the whole series the details of classification will vary with the increase of knowledge. No system has yet been accepted as final. One, which is sufficiently good for our present purpose, distributes animals among nine leading divisions. These are (1) the Protozoa, primitive animals, such as the Foraminifera and Infusoria; (2) the Cœlenterata, in which the bodycavity serves alike for circulation and digestion, a tribe which includes sponges, corals, and jelly-fish; (3) the Echinodermata or prickly-skinned animals, embracing the sea-lilies, star-fishes, sea-urchins, sea-cucumbers, and a

B

wormlike genus called Balanoglossus; (4) the Vermes or Worms; (5) the Arthropoda; (6) the Mollusca, among which are the well-known oyster, snail, and cuttle-fish; (7) the Molluscoidea, containing the mollusc-like lantern-shells, and the grouped animals of the Polyzoa, in some of which the so-called bird's-head' organs amuse the observer; (8) the Tunicata, the tunic-clad or mantled animals, comprising the Ascidians, whether tough-coated or gelatinous, and the Salpæ which roam the sea in, alternate generations solitary or connected in a chain; (9) the Vertebrata, with the important classes of fish, amphibians, reptiles, birds, and mammals.

It is with the central group of these nine that we are here concerned. So far as the name goes the Arthropoda are animals with jointed limbs. So far as the name goes, therefore, cats and dogs and vertebrates in general might belong to this division. But the name was given with reference not to the vertebrates, but to the vermes, for originally the worms and arthropods were included in a division called the Annulosa, animals of which the bodies contain several annuli, rings, metameres, somites, zonites, arthromeres, or segments, as they are variously called. These two sections of the Annulosa are now severed, and are distinguished by the circumstance that the one is, and the other is not, provided with jointed limbs.

The Arthropoda are defined as animals which have bodies composed of variously shaped segments; which have jointed appendages attached to some at least of the segments; which have (in general) a brain united to a ventral nerve-cord, or ganglionic chain, and which exhibit bilateral symmetry.

None of the other divisions will be found to possess all these characters combined. For example, in the vertebrata the nerve-cord is dorsal, in the mollusca the body is unsegmented, in the vermes there are no jointed appendages. Instances, it is true, are to be met with of arthropods which do not themselves answer the requirements of the definition, instances in which the body is unsymmetrical or unsegmented, and in which there are no articu

DEFINITION OF THE CLASS

3

lated limbs. But in all these instances there is a period of life when the creature possesses, though it subsequently loses, the characters which determine its place in classification.

Under the Arthropoda are included five classes, two of which are of very prominent importance in the economy of the world. The five classes are the Crustacea, Pycnogonida, Arachnida, Myriapoda, and Insecta. A sixth class, the Onychophora, is sometimes added for the sake of the peculiar genus Peripătus, but for the present it may be as well to give this the rank of an order among the myriapods, a class represented by the familiar but unfavoured centipede. The Arachnida contain spiders, scorpions, mites, as well as some other less commonly known groups. The Pycnogonida (or Pantopoda), the sea-spiders, at one time included in the Crustacea and at another time in the Arachnida, have some remarkable peculiarities, inasmuch as the ovaries of the female are found as a rule not in the trunk of the body, but in the thighs of the legs, and when the eggs are laid they are usually carried about not by the mother but in packets upon the oviferous feet of the male.

The Insecta are so strikingly distinguished by the special number of their legs that this class sometimes receives the name Hexapoda, the six-footed animals. Beetles, bees, bugs, flies, fleas, moths, spring-tails, earwigs, grasshoppers, and gnats, in countless profusion people the globe, sometimes disputing possession with man himself or rendering his life a burden, at other times offering him service direct or indirect of no mean value. It is in this class, and in this class only, that the present state of science reckons the number of species not only by scores of thousands but by hundreds of thousands, and even by millions. The class which stands nearest to the Insecta in the multitude of known species is that of the Crustacea, but the interval is so vast that, properly speaking, the Insecta are in this respect first with no second.

Of the numerous definitions which have been given of the Crustacea, it will be sufficient to quote two. According

to one of these, they are 'Aquatic Arthropoda, which breathe by means of gills. They have two pairs of antennæ, numerous paired legs on the thorax, and usually also on the abdomen.' This is compendious and useful. The statements clearly exclude all the other classes of the Arthropoda. They are also widely applicable among crustaceans; yet of these animals there are some which are not aquatic, some which have no gills, some which have not two pairs of antennæ, and some in which the 'paired legs on the thorax' are not numerous.

A different definition was given by Professor Alphonse Milne-Edwards in 1860, according to which the class of Crustacea comprises all the segmented animals with branchial or cutaneous respiration, in which the body is provided with jointed limbs, whether permanent or transitory.' The Insecta and Myriapoda breathe by means of the airtubes called trachea; most of the Arachnida by means either of trachea or pulmonary sacs known as fan trachea. From all these, therefore, the definition separates the Crustacea in a satisfactory manner, even though some terrestrial Crustacea combine tracheate with branchial respiration. There are, however, some subordinate members of the Arachnida, and the whole class Pycnogonida, in which the respiration is dependent on the surface of the body and not on any special organs. As it is only in recent years that the Pycnogonida have been constituted an independent class, it was no fault of a definition framed in 1860, that it included them among the Crustacea, to which they were then supposed to belong. They are in fact separated by many characters, one of which is the possession of a proboscis, which is supposed to have originated in the coalescence of the upper lip and the mandibles. So far as is known, they are all marine. animals. On the other hand, those Arachnida which have surface-respiration are apparently all air-breathers. To meet all existing requirements, then, the definition of the Crustacea may be framed in the following manner :—

They are Arthropoda without terminal proboscis, with respiration branchial or cutaneous, the latter only aquatic. It is not to be expected that any legitimate definition