Crustacea, an Archaic Group.—
A typical division of the Arthropod group, and one which was well represented from the earliest period up to the present day, is the CRUSTACEA. As the name denotes, these animals are generally invested with a strong shelly covering or “crust,” usually of horny or chitinous material, which in some forms is strengthened by deposits of phosphate of lime. Of the horny condition of the shell the groups of the bivalved Crustacea (Ostracoda) and the “water-fleas” (Entomostraca) supply notable instances; whilst the limy-structured shell is seen in the common crab. Some authorities separate the great extinct group of the Trilobites from the rest of the Crustacea; but it will here be convenient, in a preliminary study, to consider them together.
Development of Crustacea.—
The development of the lower forms of the Crustacea is interesting, from the fact that the young usually escapes from the egg in a larval state known as a “nauplius.” In this stage there are no segments to the body, and but a solitary median eye, such as may be seen in the common water-flea known to microscopists as Cyclops. The three pairs of appendages seen in this larval crustacean represent the two pairs of antennae and the jaws or mandibles of the full-grown form.
Among the higher Crustacea, however, there is no larval form; the young escaping from the egg in a more or less highly developed condition resembling the adult. The group of the Crabs, Lobsters and Shrimps (or Decapoda, i.e., having ten ambulatory feet) exhibit a larval stage in which the young form (“zoea”) has a segmented abdomen and seven pairs of appendages.
Trilobites.—
The first group of arthropods here described is that of the TRILOBITES. These were so named on account of the three-lobed form of the body. This particular feature distinguishes them from the Crustacea proper; which includes the Phyllopods (with leaf-like limbs), as the freshwater Estheria, the Ostracoda or Bivalved Water-fleas, the Barnacles or Cirripedia and the Higher Crustacea (Malacostraca), including Shrimps, Crabs, and Lobsters, of which the oldest representatives are the Pod-shrimps (Phyllocarida).
Habits of Trilobites.—
The remains of these primitive but often strikingly ornamented crustacean-like animals, the trilobites, are found in comparative abundance in the limestones, mudstones, and even the sandstones of the older sedimentary rocks of Australasia. They were amongst the most prolific types of animal life existing in the seas of Palaeozoic times, and are especially characteristic of Cambrian, Ordovician and Silurian rocks. Trilobites, as a group, seem to have adapted themselves to almost all conditions of marine life: some are found in the hardened black mud of shallow waters, whilst others are to be looked for in the limestones and excessively fine sediments of deeper waters. In all probability certain of these forms crawled over the soft, oozy sea-bed in order to obtain their food, and consequently their remains in the stratified rocks would be restricted to the fine black shales; whilst the freely swimming forms could change their habitat at will, and would be found alike in sandy or clayey deposits. As some indication of their varied habits, the eyes of trilobites differ greatly in size. They are always compound like the eye of the house-fly, though of a semi-lunar shape. In some forms the eyes are very small or even absent, whilst in others they are exceedingly large and prominent. This latter feature probably indicates their frequenting moderately deep water.