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parapodia (fig. 8) of the marine branchiate worms are the same things genetically as the "legs" of Crustacea and Insects (figs. 10 and 11). Hence the term Appendiculata was introduced by 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:—

Phylum—Appendiculata. Sub-phylum  1. Rotifera.
2. Chaetopoda.
3. Arthropoda.

The Rotifera are characterized by the retention of what appears in Molluscs and Chaetopods as an embryonic organ, the velum or ciliated prae-oral 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 important respects they are degenerate—reduced both in size and elaboration of structure.

The Chaetopoda are characterized by the possession of horny epidermic chaetae embedded in the integument and moved by muscles. Probably the chaetae 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 Chaetopoda 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 Chaetopoda the relative position of the prostomium, mouth and peristomium or first ring of the body, retains its primitive character. We do not find in Chaetopoda 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, however, 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 chitinized and turned (fellow to fellow on opposite sides) towards one another 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 Chaetopoda, 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 developed from a single line of primitive Gnathopods which arose by modification of parapodiate annulate worms not very unlike some of the existing Chaetopods. 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 Chaetopoda 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 fig. 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, 1900). The only exception to this is in the case of minute degenerate forms where the heart has disappeared altogether. The rigidity of the integument caused by the deposition of dense chitin upon it is intimately connected with the physiological activity and form of all the internal organs, and is undoubtedly correlated with the total disappearance of the circular muscular layer of the body-wall present in Chaetopods. (2) In all existing Arthropoda the region in front of the mouth is no longer formed by the primitive prostomium or head-lobe, but one or more segments, originally post-oral, with their appendages have passed in front of the mouth (prosthomeres). At the same time the prostomium and its appendages cease to be recognizable as distinct elements of the head. The brain no longer consists solely of the nerve-ganglion-mass proper to the prostomial lobe, as in Chaetopoda, but is a composite (syncerebrum) produced by the fusion of this and the nerve-ganglion-masses proper to the prosthomeres or segments which pass forwards, whilst their parapodia (= appendages) become converted into eye-stalks, and antennae, or more rarely grasping organs. (3) As in Chaetopoda, coelomic funnels (coelomoducts) may occur right and left as pairs in each ring-like segment or somite of the body, and some of these are in all cases retained as gonoducts and often as renal excretory organs (green glands, coxal glands of Arachnida, not crural glands, which are epidermal in origin); but true nephridia, genetically identical with the nephridia of earthworms, do not occur (on the subject of coelom, coelomoducts and nephridia, see the introductory chapter of part ii. of Lankester’s Treatise on Zoology).

Britannica 1911 Arthropod blood-sinus and heart development.png

After Lankester, Q. J. Mic. Sci. vol. xxxiv., 1893.

Fig. 1.—Diagram to show the gradual formation of the Arthropod pericardial blood-sinus and "ostiate" heart by the swelling up (phleboedesis) of the veins entering the dorsal vessel or heart of a Chaetopod-like ancestor. The figure on the left represents the condition in a Chaetopod, that on the right the condition in an Arthropod, the other two are hypothetical intermediate forms.

Tabular Statement of the Grades, Classes and Sub-classes of the Arthropoda.—It will be convenient now to give in the clearest form a statement of the larger subdivisions of the Arthropoda which it seems necessary to recognize at the present day. The justification of the arrangement adopted will form the substance of the rest of the present article. The orders included in the various classes are not discussed here, but are treated of under the following titles:—Peripatus (Onychophora), Centipede and Millipede (Myriapoda), Hexapoda (Insecta), Arachnida and Crustacea.

Sub-Phylum ARTHROPODA (of the Phylum Appendiculata).

Grade A. Hyparthropoda (hypothetical forms connecting ancestors of Chaetopoda with those of Arthropoda).

Grade B. Protarthropoda.

Class Onychophora.

Grade C. Euarthropoda.

Class 1. Diplopoda.
Class 2. Arachnida.
Grade a. Anomomeristica.
Grade b. Nomomeristica.
(a) Pantopoda.
(b) Euarachnida.
Ex.—Limulus, Scorpio, Mygale, Acarus.
Class 3. Crustacea.
Grade a. Entomostraca.
Ex.—Apus, Branchipus, Cyclops, Balanus.
Grade b. Malacostraca.
Ex.—Nebalia, Astacus, Oniscus, Gammarus.
Class 4. Chilopoda.
Class 5. Hexapoda (syn. Insecta Pterygota).
Ex.—Locusta, Phryganea, Papilio, Apis, Musca, Cimex, Lucanus, Machilis.

Incertae sedis—Tardigrada, Pentastomidae (degenerate forms).

The Segmentation of the Body of Arthropoda.—The body of the Arthropoda is more or less clearly divided into a series of rings,