ON THE OEIGIN OF VERTEBRATES PROM ARACHNIDS. 317 On the Origin of Vertebrates from Arachnids.1 By William Patten, Ph.D., Professor of Biology in the University of North Dakota, Grand Forks. With Plates XXIII and XXIV. " In the growth of each science, not only is correct observation needful for the formation of true theory, but true theory is needful as a preliminary to correct observation."—H. SFENCEB. 2 The " Annelid theory," after fifteen years of dexterous modelling, is now as far as ever either from fitting the facts of Vertebrate structure, or from shedding any direct light on the great problem of the origin of Vertebrates. It certainly is not without significance that, of all those who with willing eyes and minds have grappled with the Annelid theory, not one has discovered a distinctively Annelid feature in Vertebrates : mesoblastic somites, nephridia, segmental appendages, and seg- mental sense-organs are found in nearly all segmented animals. 1 I include in the Arachnida the Spiders, Scorpions, Linvulus, Trilobites, and Merostomata. 2 Most of my observations on Acdlius, Scorpio, and Limulus were made in the Lake Laboratory, Milwaukee, Wis. I am greatly indebted to the founder of that institution, Mr. E. P. Allis, for generously placing at my disposal the excellent facilities for research which his laboratory affords. As a full description of my observations could not be published without considerable delay, it seemed advisable to present my theoretical conclusions first, at the same time giving a short account of those facts bearing directly on the subject-matter. A few simple diagrams have been introduced to make the text more intelligible. These are throughout referred to as Pigs. 1, 2, 3, &c. The reference to figures in the two plates is always indicated by the addition of the letters PI. XXIII or PI. XXIV. VOL. XXXI, PART III.—NEW SER. Y 318 WILLIAM PATTEN. In failing to add materially to what the anatomy and embryology of Vertebrates themselves can demonstrate, the Annelid theory not only is sterile, but is likely to remain so; because unspecialised segmentsbeingcharacteristio of Annelids, it cannot hope to elucidate that profound specialisation of the Vertebrate head which it is the goal of Vertebrate morphology to expound. Moreover, since Vertebrate morphology itself reflects as an ancestral image only the dim outlines of a segmented animal—hut still not less a Vertebrate than any now living,—it is clear that the problem must be solved, if at all, by the discovery of some form in which the specialisation of the Vertebrate head is already foreshadowed. Since of all Invertebrates, concentration and specialisation of head segments is greatest in the Arachnids, it is in these, on a priori grounds, that we should expect to find traces of the characteristic features of the Vertebrate head. Finding from time to time confirmation of this preconceived idea as the unexpected complexity of the Arachnid cephalothorax revealed itself, I now feel justified in formulating a theory that Verte- brates are derived from Arachnids. I have presented the facts as they appear to me, and have hazarded an interpretation of them; not, however, without a lively sense of the difficulties of the task, certainly not without the conviction that I may have fallen into errors which greater experience and a better knowledge of the intricacies of Vertebrate anatomy might have avoided. In the following preliminary sketch of the structure of Limulus, and especially of the Scorpion, I shall attempt to prove—(1) That in the Scorpion the cephalothoracic neuro- meres, nerves, sense-organs, and mesoblastic somites present, in a general way, not only the same specialisation and the same numerical arrangement in groups, but also the same difference as a whole from the body-segments, as do the corresponding parts in the Vertebrate head; (2) that the Arachnid cartila- ginous sternum represents the primordial cranium of Verte- brates ; (3) that in the Trilobites and Merostomata the internal structure of the cephalothorax resembles in some respects that ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 319 of Scorpio and Limulus; (4) that the remarkable fish-like Pterichthys and related forms, judging from their external structure, are closely related to*the Merostomata, and serve to connect Arthropods with Vertebrates; and (5) that the em- bryology of Vertebrates in its main features cau be reduced to the Arthropod type. First let me state certain conclusions that have been reached concerning segmentation in Arthropods. In Scolopendra each neuromere has four pairs of spinal nerves j the first two pairs in each neuromere are larger and darker, and probably contain more sensory fibres than the two following pairs. Certain facts indicate that this condition is the ground plan of the nervous system in all Arthropods, and that the various modifications of it found in other Arthropods are produced by fusion of the nerves. The two sensory nerves tend to fuse with each other first; afterwards the two motor nerves; and finally the double motor and the double sensory nerves unite, thus producing, in different groups of Arthropods, neuromeres with four, three, two, and one pair of nerves. In Scolopendra the neuromeres appear to be double; and, if what we have indicated above is true, it follows that in all Arthropods the neuromeres, and consequently the segments themselves, are double. In support of this view we mention the following facts :—(1) In all Arthropods carefully studied two cross commissures have been found in each neuromere. (2) In Acilius the median furrow between these cross com- missures is similar to that between the successive neuromeres. (3) In Acilius, according to my observations, there are two pairs of tracheal invaginations in each segment: one pair, that which is always readily seen, is situated near the anterior edge of the segment; the other, which is very rudimentary and difficult to distinguish, is situated in the same line as the first, but near the posterior edge of the segment. (4) In all the insect embryos I have examined, and in almost all figures where the tracheal openings were represented, the stigmata were situated near the anterior edge of the segment. (5) The 320 WILLIAM PATTEN. frequent presence in Arthropods, especially Crustacea, of bifurcated appendages; this condition is due, we may suppose, to the partial fusion of two "originally distinct appendages. (6) The frequent occurrence of insect monsters having double pairs of legs. (7) According to Heathcote's important obser- vations, the segments in Julus are certainly double, as shown by the duplication in each segment of the somites, cardiac ostia, arteries, neuromeresj tracheae, and legs. (8) In Scorpio the neuromeres are distinctly double, each one being composed of a large anterior portion and a small posterior one. Large pit- like invaginations of the median furrow are found between the halves of the anterior portions, and faint indications of a second series of pits between the halves of the posterior por- tions (PL XXIV, fig. 3). But in Scorpio the most singular feature of all is that the parts of each abdominal neuro- mere finally separate, the posterior portions uniting with the anterior portion of the neuromere just behind it (PI. XXIV, figs. 3 and 4, and Fig. 11, p. 348). This process may be followed with ease and perfect certainty in surface views. All these facts point to the conclusion that the segments in all Arthropods are double, and are derived from those of diplopod-like ancestors. If Vertebrates are derived from Arthropods, they are also, in all probability, composed of double segments. It may be worth mentioning in this connection that in many fishes the spinal nerves, and especially the cervical ones, split up into two, three, and sometimes four pairs of nerves for each neuro- mere. I. THE GROUPING OF THE CRANIAL NEUROMERES OF SCOR- PIO is a result of the varying union of the first thirteen neuromeres. From the cephalic lobes three neuromeres arise which fuse completely to form the fore-brain of the adult (figs. 1—4, Pis. XXIII and XXIV, and Fig. 1, F. B., p. 321). The first neuromere of the six thoracic segments pushes its PIG. 1.—Brain of adult Scorpion, constructed by means of sections and dissections.—A. B. Accessory brain. H. B. Hind-brain. M. B. Mid-brain. F. B. Pore-brain. a. co. Anterior commissures. a. h. »'-c. Anterior li£emal nerves of the thoracic neuromeres. c. sy. Cranial sympathetic (?). g1. Large ganglion derived from the segmental and coxal sense-organs, and giving off many branches to the scattered sense-organs in skin of chelae, g.f. Ganglion fusiforme. g. nod. Ganglion nodosum, or g. striatum. h. n3. A very delicate root, probably representing the haemal nerve of the third fore-brain neuromere, partially fused with tbe haemal nerve of the cheliceral segment, h. v. and h. v". The independent anterior and posterior nerves of the first vagus neuromere. h. »a. and h. vs. The four partially fused haemal nerves of the second and third vagus neuromeres. 1. e. n. Lateral eye nerve, n. Mouth, m. e. n. Median eye nerve, m. si. n. Median stomodseal or rostral nerve, n. e. Comb or pectinal nerve. n. #'-6. Neural ganglia at the base of the neural nerves, n. w1-6. Neural nerves of the thoracic 1 1 segments. A. v -*. Roots of the neural vagus nerves, p. h. n. -". Posterior haemal nerves of the thorax, p. co. Posterior brain commissure. *. /. Semilunar lobe, or first brain-segment, r. m. Ketractor muscles to oesophagus. 322 WILLIAM PATTEN. way in front of the mouth, forming a sharply denned region, that I shall call the mid-brain (Fig. 1, M. B.). The remaining five thoracic neuromeres are imperfectly fused; they constitute the hind-brain (H.