STRUCTURE AND CLASSIFICATION OF THE AIITHHOPODA. 523 The Structure and Classification of the Arthropoda. By E. Ray Lankcster, M.A., LX.D., F.R.S., Director of the Natural History Departments of the British Museum. With PUe 42. [BY the great kindness of the proprietors of the tenth edi- tion of the 'Encyclopaedia Britannica' I have received per- mission to reprint in this journal the articles ARTHROPODA and ARACHNIDA, which I contributed to its pages. I have been anxious that morphologists should consider the views which I have put forward in these articles (written now nearly four years ago). At the same time I have observed that they have entirely escaped the notice of two authors who have recently written general essays on the Arthropoda, viz. Dr. A. S. Packard, of Salem, Mass., and Mr. G. H. Carpenter, of Dublin. I have revised both articles only in regard to verbal inaccuracies, excepting where I have definitely stated that new matter is introduced. I hope that in their present form these articles will not fail to come under the notice of s.—E. R. L.] ARTHROPODA is the name of one of the three sub-phyla into which one of the great phyla (or primary branches) of 524 E. EAY LANKBSTER. coeloinoccelous animals—the Appendiculata—is divided, the other two being respectively the Ch^topoda and the Rotifera. The word " Arthropoda" was first used in classification by Siebold and Stannius (' Lehrbuch der vergleich. Auatomie/ Berlin, 1845) as that of a primary division of animals, the others recognised in that treatise being Protozoa, Zoophyta, Vermes, Mollusca, and Vertebrata. The names Oondylopoda and Gnathopoda have been subsequently proposed for the same group. The word refers to the jointing of the chitinised exo-skeleton of the limbs or lateral appendages of the animals included, which are, roughly speaking, the Crustacea, Arach- nida, Hexapoda (so-called "true insects"), Centipedes, and Millipedes. This primary group was set up to indicate the residuum of Cuvier's Articnlata when his class Annelides (the modern Cheefcopoda) was removed from that " embranche- ment." At the same time Siebold and Stannius renovated the group Vermes of• Linuasus, and placed in it the ChEefcopods and the parasitic worms of Cuvier, besides the Rotifers and Turbellarian worms.1 1 As a matter of fact the group Arthropoda itself, thus constituted, was precisely identical in its area with the class Insecta of Linnaeus, the Entoma of Aristotle. But by causes which it is not easy to trace the word "Insect" had become limited since the days of Linnaeus to the Hexapod Pterygote forms, to the exclusion of his Aptera. Lamarck's penetrating genius is chiefly responsible for the shrinkage of the word Insecta, since it was lie who, forty years after Linnseus's death, set up and named the two great classes Crustacea and Araclmida (included by Linnseus under Insecta as the order "Aptera") assigning to them equal rank with the remaining Insecta of Linneeus, for which he proposed the very appropriate class-name " Hexapoda." Lamarck, however, appears not to have insisted on this name Hexapoda, and so the class of Pterygote Hexapods came to retain the group-name Insecta, which is, historically or etymological ly, no more appropriate to them than it is to the classes Crustacea and Araclmida. The tendency to retain the original name of an old and comprehensive group for one of the fragments into which such group becomes divided by the advance of knowledge—instead of keeping the name for its logical use as a comprehensive term, including the new divisions, each duly provided with a new name—is most curiously illustrated in the history of the word Physiology. Cicero says, "Physiologia naturae ratio," and such was the meaning of the name Physiologus, given to a STOUCTGBE AND CLASSIFICATION OF THE AllTHROPODA. 525 The result of the knowledge gained in the last quarter of the nineteenth century has been to discredit altogether the group Vermes, thus set up and so largely accepted by German writers even at the present day. We have, in fact, returned very nearly to Cuvier's conception of a great division or branch, which he called Avticnlata, including the Arthropoda and the Chtetopoda (the latter equivalent to the Annelides of Lamarck, a name adopted by Guvier), and differing from it only by the inclusion of the Rotif era. The name Articulata, introduced by Cuvier, has not been retained by subsequent writers. The same, or nearly the same assemblage of animals has beeu called Entomozoariaby DeBlaiuville (1882), ArthrozoabyBurmeister (1843), Entornozoa or Anuellata by Milne-Edwards (1855), and Annulosa byM'Leay (1819), who was followed by Huxley (1856). The character pointed to by all these terms is that of a ring-like segmentation of the body. This, however, is not the character to which we now ascribe the chief weight as evidence of the genetic affinity and monophyletic (uni- ancestral) origin of the Chastopods, Rotifers, and Arthropods. It is the existence in each ring of the body of a pair of hollow lateral appendages or parapodia, moved by intrinsic muscles and penetrated by blood-spaces, which is the leading fact indicating the affinities of these great sub-phyla, and uniting them as blood relations. The pai-apodia (fig. 7) of the marine branchiate worms are the same things genetically as the "legs" of Crustacea and insects (fags. 9 and 10). Hence the term Appendiculata was introduced by Lankester cyclopaedia of wliat was kuown and imagined about earth, sea, sky, birds, beasts, and fishes, which for a thousand years was the authoritative source of information on these matters, and was translated into every European tongue. With the revival of learning, however, first one and then another special study became recognised—anatomy, botany, zoology, mineralogy, until at last the great comprehensive term Physiology was bereft of all its once-included subject-matter excepting the study of vital processes pursued by the more learned members of the medical profession. Professional tradition, and an astute perception on iheir part of the omniscience suggested by the terms, have left the medical men in English-speaking lands in undisturbed but illogical possession of the words physiology, physic, and physician. 526 E. RAY LANKESTER. (preface to the English edition of Gegenbaur's ' Comparative Anatomy,' 1878) to indicate the group. The relationships of the Arthropoda thus stated are shown in the subjoined table : f Sub-phylum 1. Rotifera. Phylum APPENDICDLATA^ „ 2. Chsetopoda. I ,, 3. Arthropoda. The Rotifera are characterised by the retention of what appears in Molluscs and Chaatopods as an embryonic organ, the velum or ciliated prasoral girdle, as a locomotor and food-seizing apparatus, and by the reduction of the muscular parapodia to a rudimentary or non-existent condition in all present surviving forms except Pedalion. In many im- portant respects they are degenerate—reduced both in size and elaboration of structure. The Chastopoda are characterised by the possession of horny epidermic chsetas embedded in the integument and moved by muscles. Probably the chaatse preceded the development of parapodia, and by their concentration, and that of the muscular bundles connected with them at the sides of each segment, led directly to the evolution of the parapodia. The parapodia of Chtetopoda are never coated with dense chitin, and are, therefore, never converted into jaws; the primitive "head-lobe" or prostomium persists, and frequently carries eyes and sensory tentacles. Further, in all members of the sub-phylum Chastopoda the relative position of the prostomium, mouth, and peristomium or first ring of the body retains its primitive character. We do not find in Chastopoda that parapodia, belonging to primitively post-oral rings or body-segments (called " somites," as proposed by H. Milne-Edwards), pass in front of the mouth by adaptational shifting of the oral aperture. (See, how- ever, 8.) The Arthropoda might be better called the " Gnathopoda/' since their distinctive character is that one or more pairs of appendages behind the mouth are densely chitinised and turned (fellow to fellow on opposite sides) towards one STBnCTUBB AND CLASSIFICATION OF THE ARTHROPODA. 527 auother so as to act as jaws. This is facilitated by an important general change in the position of the parapodia; their basal attachments are all more ventral in position than in the Chtetopoda, and tend to approach from the two sides towards the mid-ventral line. Very usually (but not in the Onychophora = Peripatus) all the parapodia are plated with chitin secreted by the epidermis, and divided into a series of joints—giving the " arthropodous" or hinged character. There are other remarkable and distinctive features of structure which hold the Arthropoda together, and render it impossible to conceive of them as having a polyphyletic origin,—that is to say, as having originated separately by two or three distinct lines of descent from lower animals; and, on the contrary, establish the view that they have been deve- loped from a single line of primitive Gnathopods which arose by modification of parapodiate annulate worms not very unlike some of the existing ChEetopods. These additional features are the following:—(1) All existing Arthropoda have an ostiate heart and have undergone " phleboedesis," that is to say, the peripheral portions of the blood-vascular system are not fine tubes as they are in the Cheetopoda and as they were in the hypothetical ancestors of Arthropoda, but are swollen so as to obliterate to a large extent the coelom, whilst the separate veins entering the dorsal vessel or heart have coalesced, leaving valvate ostia (see Pig. 1*) by which the blood passes from a pericardial blood-sinus formed by the fused veins into the dorsal vessel or heart (see Lankester's 'Zoology,' part ii, introductory chapter; A.