From the Limulava to the Eurypterida is a long leap, and before it can be made without danger, many intermediate steps must be placed in position. The direct ancestor of the Eurypterida is certainly not to be seen in the highly specialized Sidneyia, and probably not in Emeraldella, but it might be sought in a related form with a few more segments. The few species now known do suggest the beginning of a grouping of appendages about the mouth, a suppression of appendages on the abdomen, and a development of gills on the thorax only. Further than that the route is uncertain.
Clarke and Ruedemann, whose recent extensive studies give their opinion much weight, seem fully convinced that the Merostomata could not have been derived from the Trilobita, but are rather inclined to agree with Bernard that the arachnids and the crustaceans were derived independently from similar chætopod annelids (1912, p. 148).
The greater part of their work was, however, finished before 1910, and although they refer to Walcott's description of the Limulava (1911), they did not have the advantage of studying the wonderful series of Crustacea described by him in 1912. While the evidence is far from clear, it would appear that the discovery of animals with the form of Limiting and the eurypterids and the appendages of trilobites means something more than descent from similar ancestors. Biramous limbs of the type found in the trilobites would probably not be evolved independently on two lines, even if the ancestral stocks were of the same blood.
The Aglaspidæ, as represented by Molaria and Habelia in the Middle Cambrian, are quite obvious closely related to the trilobites easily derived from them, and retain numerous of their characteristics. That they are not trilobites is, however, shown by the presence of two pairs of antennæ, the absence of facial sutures, and the possession of a spine-like telson.
The Aglaspidæ have always been placed in the Merostomata, and nearer the Limulidæ than the Eurypterida. The discovery of appendages does not at all tend to strengthen that view, but indicates rather that they are true Crustacea which have not given rise to any group now known. The exterior form is, however, Limulus-like, and since it is known from ontogeny that the ancestor of that genus was an animal with free body segments, there is still a temptation to try to see in the Aglaspidæ the progenitors of the limulids.
The oldest known Limulus-like animal other than the Aglaspidæ is Neolimulus falcatus Woodward (Geol. Mag., dec. 1, vol. 5, 1868, p. 1, pl. 1, fig. 1). The structure of the head of this animal is typically limuloid, with simple and compound eyes and even the ophthalmic ridges. Yet, curiously enough, it shows what in a trilobite would be considered the posterior half of the facial suture, running from the eye to the genal angle. The body is composed of eight free segments with the posterior end missing. Belinurus, from the Mississippian and Pennsylvanian, has a sort of pygidium, the posterior three segments being fused together, and Prestwichia of the Pennsylvanian has all the segments of the abdomen fused together. So far as form goes, a very good series of stages can be selected, from the Aglaspidæ of the Cambrian through Neolimulus to the Belinuridæ of the late Palæozoic and the Limulidæ of the Mesozoic to recent. Without much more knowledge of the appendages than is now available, it would be quite impossible to defend such a line. It is, however, suggestive.
The trilobites were such abundant and highly variable animals, adapting themselves to various methods of life in the sea, that it appears highly probably that some of them may have become adapted to life on the land. The ancestors of the Chilopoda, Diplopoda, and Insecta appear to have been air-breathing animals as early as the Cambrian, or at latest, the Ordovician. Since absolutely nothing is yet known of the land or even of the fresh-water life of those periods, nothing can now be proved.
In discussing the relationship of the trilobites to the various tracheate animals, I have pointed out such palæontologic evidence as I have been able to gather. Studies in the field of comparative morphology do not fall within my province. I only hope to have made the structure of the trilobite a little more accessible to the student of phylogenies.