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ARTHROPODA

were ancestrally post-oral, but have become prae-oral “ by adaptational shifting of the oral aperture.” This has proved to be a sound hypothesis and is now accepted as the basis upon which the Arthropod head must be interpreted (see Korschelt and Heider (3)). Further, the morphologists of the ’fifties appear, with few exceptions, to have accepted a preliminary scheme with regard to the Arthropod head and Arthropod segmentation generally, which was misleading and caused them to adopt forced conclusions and interpretations. It was conceived by Huxley, among others, that the same number of cephalic somites would be found to be characteristic of all the diverse classes of Arthropoda, and that the somites, not only of the head but of the various regions of the body, could be closely compared in their numerical sequence in classes so distinct as the Hexapods, Crustaceans, and Arachnids. The view which it now appears necessary to take is, on the conFlo 2 Diag am of t e lR ad ',i '7 f • ^ ^ 1 trary, this—viz., that all the Arciiset ciiaetopod. Letters as thropoda are to be traced to a t! 'iirostomkd “entade;° common ancestor resembling a parapodium. (From Good- Chsetopod worm, but differing from it in having lost its chsetse and in having a prosthomere in front of the mouth (instead of prostomium only) and a pair of hemignaths (mandibles) on the parapodia of the buccal somite. From this ancestor Arthropods with heads of varying degrees of complexity have been developed characteristic of the different classes, whilst the parapodia and somites of the body have become variously modified and grouped in these different classes. The resemblances which the members of one class often present to the members of another class in regard to the form of the limb-branches (rami) of the parapodia, and the formation of tagmata (regions) are not hastily to be ascribed to common inheritance, but we must consider whether they are not due to homoplasy—that is, to the moulding of natural selection acting in the different classes upon fairly similar elements under like exigencies. The structure of the head in Arthropods presents three profoundly separated grades of structure dependent upon the number of prosthomeres which have been assimilated by the prae-oral region. The classes presenting these distinct plans of head-structure cannot be closely associated in any scheme of classification professing to be natural. Peripatus, the type-genus of the class Onychophora, stands at the base of the series with only a single prosthomere (Fig. 3). In Peripatus the prostomium of the Clnetopodlike ancestor is atrophied, but it is possible that two processes on the front of the head (FP) represent in the embryo the dwindled prostomial tentacles. The single prosthomere carries the retractile tentacles as its “parapodia.” The second somite is the buccal somite (II, Fig. 3); its parapodia have horny jaws on their ends, like the claws on the following legs (Fig. 8), and act as hemignaths (mandibles). The study of sections of the embryo establishes these facts beyond doubt. It also shows us that the neuromeres, no less than the embryonic coelomic cavities, point to the existence of one, and only one, prosthomere in Peripatus, of which the “ Protocerebrum,” P, is the neuromere, whilst the Deuterocerebrum, D, is the neuromere of the second or buccal somite. A brief indication of these facts is given by saying that the Onychophora are “ deuterognathous ”—that is to say, that the buccal somite carrying the mandibular hemignaths is the second of the whole series.

What has become of the nerve-ganglion of the prostomial lobe of the Chaetopod in Peripatus is not clearly ascertained, nor is its fate indicated by the study of the embryonic head of other Arthropods so far. Probably it is fused with the Proto- CLnt cerebrum, and may also be concerned in the history of the very peculiar paired eyes of Peripatus, which are like those of Chsetopods in structure—viz., vesicles with an intra-vesicular lens, whereas the eyes of all other Arthropods have essentially another structure, being Fig. 3.—Diagram of the head and ad“ cups ” of the epidermis, in jacent region of Peripatus. Monoprosthomerous. m, Mouth ; I, coelom of which a knob-like or rod-like the first somite which carries the antennae is in front of the mouth ; thickening of the cuticle is II, coelomand of the second somite which fitted as refractive medium. carries the mandibles (hence deuteroIII and IV, coelom of the In Diplopoda ( Julus, &c.) gnathous); third and fourth somites ; FP, rudithe results of embryological mentary frontal processes perhaps the prostomial tentacles study point to a composi- representing of Polychseta ; Ant, antenna or tactile Md, mandible; Op, oral tion of the front part of the tentacle; papilla; P, protocerebrum or forehead exactly similar to that most cerebral mass belonging to the first somite; D,deuterocerebrum, conwhich we find in Onycho- sisting of ganglion cells belonging to phora. They are deutero- the second or mandibular somite. (After Goodrich.) gnathous. The Arachnida present the first stage of progress. Here embryology shows that there are two prosthomeres (Fig. 4), and that the gnathobases of the chelae which act as the first pair of hemignaths, are carried by the third somite. The Arachnida are therefore tritognathous. The two prosthomeres are indicated by their coelomic cavities in the embryo (I and II, Fig. 4), and by two neuromeres, the Protocerebrum and the Deuterocerebrum. The appendages of the first prosthomere are not present as tentacles, as in Peripatus and Diplopods, but are possibly represented by the eyes or possibly altoE gether aborted. The appenQh '1 dages of the second prosthomere are the well-known chelicerse of the Arachnids, rarely, if ever, antenniform, but modified as “retroverts” or clasp - knife fangs in spiders. The Crustacea (Fig. 5) and the Hexapoda (Fig. 6) jacent region of an Arachnid. Dipros- agree 1U having three SOUliteS thomerous in the adult condition, in fr0nt of the mouth, and though embryologically the append-. . i it , ’ ages of somite II and the somite it IS probable, thoughn not itself are, as here drawn, not actually aSCC1 o .p .4-nir,prI fhof +L0 Philn in front of the mouth; E, lateral fl f T t(lIne G, that the L-illlOeye; Ch, chelicera; to, mouth; P, poda (Scolopendra, etc.) are protocerebrum; D, deuterocerebrum ; • . m-, b I, ii, in, iv, coelom of the first, tn the same case. The three second third and fourth somites, prosthomeres or prse - oral 1 (After Goodrich.) .,, somites ofp Crustacea due to the sinking back of the mouth one somite farther than in Arachnida are not clearly indicated by coelomic cavities in the embryo, but their existence is clearly established by the development and position of the appendages and by the neuromeres. The eyes in some Crustacea are mounted on articulated stalks, and from the fact that they can after injury be replaced by antenna-like appendages, it is inferred that they represent the parapodia of the most anterior prosthomere. The second prosthomere carries the first pair of antennae and the third the second pair of antennae. Sometimes this pair of appendages has not a merely tactile jointed ramus, but is converted into a claw or