Fig. [142].—Triarthrus becki, Green, × 2½. Utica Slate (Ordovician), near Rome, New York. A, Ventral surface with appendages; Ep, metastome; Hy, hypostome. B, second thoracic appendage; en, endopodite; ex, exopodite, × 12. (After Beecher.)
In Triarthrus each segment, except the anal, bears a pair of appendages, all of which, except the first, are biramous. There are five pairs of cephalic appendages; the first pair are attached at each side of the hypostome, and have the structure of antennae, each consisting of a single flagellum formed of short conical joints. The other cephalic appendages increase in size successively. At present the second and third pairs are not satisfactorily known, but appear to have been similar to the fourth and fifth pairs. The second pair is attached at the level of the posterior end of the hypostome. The fourth and fifth pairs have large, triangular coxopodites which served as gnathobases, their inner edges being denticulate; the endopodites consist of stout joints; the exopodites are slender, and bear setae which are often arranged in a fan-like manner.
The first pair of appendages appear to be antennules, whilst the second pair probably represent the antennae, the third pair the mandibles, and the fourth and fifth pairs the maxillae of other Crustacea. The appendages of the thorax and pygidium do not differ essentially from the two posterior cephalic appendages. Those on the anterior part of the thorax are the longest; the others gradually decrease in size in passing posteriorly. Each thoracic leg (Fig. [142], B) consists of a short coxopodite with an inward cylindrical prolongation forming a gnathobase which is best developed on the anterior legs; the endopodite and exopodite are long and nearly equal; the former consists of six joints tapering gradually to the end; the latter, of a long proximal joint with a denticulate edge and a distal part of ten or more joints, and it bears a row of setae along the whole of the posterior edge.
The anterior appendages of the pygidium differ but little from the posterior thoracic legs; but the phyllopodous character, which appears in the latter, becomes more distinct in the appendages of the pygidium, especially those near its posterior end, and is due to the broad, flat, laminar joints of the endopodite.
The more striking features of the appendages of Triarthrus are the small amount of differentiation which they show in different parts of the body, especially the want of specialisation in the cephalic region; the distinctly biramous character of all except the first pair; and the presence of one pair of functional antennae only, and the occurrence of thoracic gnathobases.
In Trinucleus the appendages are not so well known, but they are considerably shorter than in Triarthrus.
In the Palaeozoic rocks of Bohemia, where Trilobites are very perfectly preserved, Barrande[[193]] discovered the larval forms of several species, and in some cases was able to trace out the development very completely, but in others the earliest stages were not found. In the strata in which Trilobites occur Barrande found minute spheroidal bodies, usually of a black colour, and only a little smaller than the youngest larval stages; those bodies are probably the eggs of Trilobites. Since the publication of Barrande’s work the development of some species found in North America has been studied by Ford, Matthew, Walcott, and Beecher. But even now the development is known in only a very small proportion of the total number of genera of Trilobites. The early larval form (Fig. [143], A) is similar in general character in the various species in which it has been found. It is circular or ovoid in outline, with a length of from 0·4 to 1 mm., and consists of a large cephalic and a small pygidial portion; the axis is distinct and usually shows more or less clear indications of five cephalic segments; the eyes, when present, are found at or near the front margin, and the free cheeks, if visible at all on the dorsal surface, are narrow. For this early larval form Beecher has proposed the name “protaspis”; he regards it as the representative of the Nauplius of other Crustacea, but that view is not accepted by Professor J. S. Kingsley.[[194]]
The general changes which occur in the course of development are: modifications in the shape and relative size of the glabella, and of the number and depth of the glabella-furrows; the growth of the free cheeks and the consequent inward movement of the facial sutures and eyes; the introduction of and gradual increase in number of the thoracic segments, and the relative decrease in size of the head.
Fig. [143].—Development of Sao hirsuta, Barr. Cambrian. A, Protaspis; B-F, later stages; G, adult. The small outlines below each figure show the actual size of each specimen. (After Barrande.)