a, Paradoxides. b, Dikellocephalus. c, Conocoryphe (head). d, Agnostus (head and tail).
The remarkable group of the Trilobites had precedence in order of time of the Nautiloid shell-fishes. No animal structures can well be more dissimilar than those of the two great groups of aquatic animals which popular speech confounds under the name of “shell-fishes.” Take a whelk and a crab, for example, and compare their general forms, the structure of their shells, and their organs of motion, and it is scarcely possible to imagine any two animals more unlike; and when we examine their anatomy in detail this difference does not diminish. They have, it is true, corresponding parts, and these parts serve similar uses, but in plan of structure they are wholly different. Yet both animals may live in the same pool, and may subsist on nearly the same food. If we attempt to find some common type which both resemble, we may trace the structure of the crab back to those of some of the marine worms with which it has some affinity, and those of the whelk to such creatures as the Lingula, which are supposed to have a resemblance to the worms. But still the two types, that of the Mollusk and the Articulate, are distinct even from their first appearance in the egg, nor have either any close affinities with the Protozoa, the Hydroids, or the Corals.
Fig. 76.—Transverse section of Calymene. A Silurian Trilobite.—After Wolcott.
a, Dorsal shell. b, Visceral cavity. c, Legs. d, Epipodite—gill-cleaner or palp. e, Spiral gills.
Both types meet us in the Early Cambrian, but while the Mollusk is there represented only by low forms, the Articulate is then not only in the humble guise of the worm, but in the complex and highly organised form of the Trilobite ([Figs. 28 and 75]). What older phases they may have passed through we know not; but in the Lower Cambrian we have various forms of these animals, including some of the largest known as well as some of the smallest; some of the most complex in number of parts as well as some of the simplest. These animals, in short, seem to have appeared at once all over the world fully formed, and in a variety of generic and specific forms; and nothing short of a very large faith in the imperfection of the geological record can suffice to account for their evolution.
Fig. 76a.—Burrows of Trilobite and of modern King-crab. The Trilobite burrow is known as Ruschinites, and has been supposed to be a sea-weed of the kind called Bilobites.