BUNODES. 29 



It is thus clear that the position of the animal during locomotion has no 

 morphological value whatever. It is necessary to emphasize this point, because 

 the ancient superstition, to the effect that it is always the same surface of a verte- 

 brate or of an invertebrate that points heavenward, or that it is the baptismal name 

 of a surface that determines its identity, is still deeply rooted in the minds of an 

 incredible number of zoologists. 



II. COMPARISON OF ADULT ARTHROPODS AND VERTEBRATES. 



Bunodes. The form that perhaps most nearly realizes the generalized 

 arachnid type we have tried to portray is Bunodes, a small, silurian merostome 

 from the island of Oesel, Russia. (Fig. 13.) In my visit to this island in 1901, 

 a large collection of these forms was obtained from which I have made a large 

 scale model, showing in detail the essential features of the haemal surface. This 

 animal is remarkable for the fact that it has no recognizable exoskeleton. The 

 fossils consist of well denned, but very thin, carbonaceous films, in a fine chalky 

 matrix. They are found side by side with small eurypterids that are covered 

 with a delicate chitenous membrane, still retaining apparently its original, chem- 

 ical structure, and close to fragments of Tremataspis, consisting of perfectly 

 preserved, calcareous, dermal plates. It is therefore probable that Bunodes had 

 neither a chitenous nor a calcareous exoskeleton. 



The general form of the body is intermediate between that of Limulus and 

 that of a trilobite, or of a typical merostome. All the five head divisions, except 

 the diacephalon, are clearly indicated, and they are surprisingly like those in 

 larval Limuli (Fig. 152). There is a distinct procephalon, six thoracic, two vagal 

 (chelarial and opercular?), and five branchial metameres. The most remarkable 

 feature is a pair of short, slender antennas clearly seen in one specimen. 



For the sake of exposition we may picture to ourselves the manner in which 

 an adult arachnid, or other arthropod, might be moulded into a vertebrate, although 

 it is manifestly impossible for any adult animal to be converted into another. 

 We may start with a form like Limulus, or Bunodes, or an eurypterid, or with an 

 adult phyllopod, like Branchipus, or a cladoceran, or cirriped. 



In practically all these animals, extensive lateral, or pleural folds develop 

 on the sides of the cephalothorax, that either extend in a nearly horizontal plane, 

 to form a broad, shield-shaped cephalothorax, with backwardly directed cornua, 

 as in the marine arachnids (Fig. 155), or the folds may be directed toward the 

 neural surface, forming, in extreme cases, the bivalve shield, or mantle, of phyllo- 

 pods (Fig. 273), ostracoda (Fig. 307), cladocera (Figs. 8 and 9), and cirripeds 

 (Fig. 275). It may enclose the head, or the entire body, in a large peribranchial, 

 or atrial chamber, which contains, or into which opens, the nutrient, excretory, 

 respiratory, and genital organs. Another characteristic feature is the often 

 enormous labrum, or rostrum, that shows a persistent tendency to migrate back- 

 ward, forming an overhanging lid to the mouth (Fig. 7). The rostrum and the 



