1. The appendages, both exopodites and endopodites, are relatively feebly developed as swimming organs. This has been discussed above, and need not be repeated. It must in fairness be observed, however, that many modern Crustacea get about very well with limbs no better adapted for swimming than those of the trilobites.
2. The articulations of the thoracic segments with each other and with the two shields are such as to allow the pygidium to swing through an arc of at least 270, that is, from a position above the body and at right angles to it, around to the plane of the bottom of the cephalon. Specimens are occasionally found in which the thorax and pygidium are so flexed that the latter shield stands straight above the body. A well preserved Dipleura in this position is on exhibition in the Museum of Comparative Zoology, and Mr. Narraway and I have figured a Bumastus milleri in the same attitude (Ann. Carnegie Mus., vol. 4, 1908, pl. 62, fig. 3).
3. What little can be learned of the musculature (see under musculature, seq.) indicates that the trilobites had powerful extensor and flexor muscles, such as would be required for this method of swimming. It may be objected that the longitudinal muscles were too small to permit the use of a caudal fin. In the lobster, where this method of progression is most highly developed, there is a large mass of muscular tissue which nearly fills the posterior segments. Trilobites have not usually been thought of as powerfully muscled, but it may be noted that in many cases broad axial lobes accompany large pygidia. As the chief digestive region appears to have been at the anterior end, and other organs are not largely developed, it seems probable that the great enlargement of the axial lobe was to accommodate the increased muscles necessary to properly operate the pygidium. It may be noted that in all these genera the axial lobe of the pygidium is either short or narrow.
4. The geological history of the rise of caudalization favors this view. With the exception of the Agnostidæ and Eodiscidæ, all Lower Cambrian trilobites had small pygidia, and the same is true of those of the Middle Cambrian of the Atlantic realm (except for the Dorypyge of Bornholm). In Pacific seas, however, large-tailed trilobites of the families Oryctocephalidæ, Bathyuridæ, and Asaphidæ then began to be fairly common, though making up but a small part of the total fauna of trilobites. In the Upper Cambrian of the Atlantic province the Agnostidæ were the sole representatives of the isopygous trilobites, while in the Pacific still another family, the Dikelocephalidæ, was added to those previously existing.
With the Ordovician, caudalization reached its climax and the fashion swept all over the world. It is shown not so much in the proportion of families with large pygidia, as in the very great development of the particular trilobites so equipped. Asaphidæ and Illænidæ were then dominant, and the Proëtidæ, Cyclopygidæ Goldiidæ, and Lichadidæ had begun their existence. A similar story is told by the Silurian record, except that the burden of the Asaphidæ has been transferred to the Lichadidæ and Goldiidæ. All the really old (Cambrian) families of trilobites with small pygidia had now disappeared. In the general dwindling of the subclass through the Devonian and later Palæozoic, the few surviving species with small pygidia were the first to go, and the proëtids with large abdominal shields the last.
The explanation of this history is probably to be found in the rise of the predatory cephalopods and fishes, the natural enemies of the trilobites, against whom they could have no other protection than their agility in escaping. While the records at present known carry the fishes back only so far as the Ordovician (fishes may have arisen in fresh waters and have gone to sea in a limited way in the Ordovician and more so in Silurian time) and the cephalopods to the Upper Cambrian, the rise of the latter must have begun at an earlier date, and it is probably no more than fair to conjecture that the attempt to escape swimming enemies caused an increase in the swimming powers of the trilobites themselves. At any rate, the time of the great development of the straight cephalopods coincided with the time of greatest development of caudalization; both were initiated in the Pacific realm, and both spread throughout the marine world during the middle Ordovician. And since, in the asaphids, a decrease in swimming power of the appendages accompanied the increase in the size of the pygidium, it seems probable that the swimming function of the one had been transferred to the other. A high-speed, erratic motion which could be produced by the sudden flap of a pygidium would be of more service in escape than any amount of steady swiftness produced by the oar-like appendages of an animal of the shape of a trilobite.
The primary function of the endobases of the coxopodites was doubtless the gathering, preparation, and carrying of food to the mouth. Although the endobases of opposite sides could not in all cases meet one another, they were probably spinose or setiferous and could readily pass food from any part of the axial groove forward to the mouth, and also send it in currents of water. The endobases of the cephalic coxopodites were probably modified as gnathites in all cases, but little is known of them except in Triarthrus, where they were flattened and worked over one another so as to make excellent shears for slicing up food, either animal or vegetable. In some cases the proximal ends of opposed gnathites were toothed so as to act as jaws, but a great deal still remains to be learned about the oral organs of all species.