The Arthropoda are all distinguished from the Chaetopoda by the fact that the head consists of one or more somites which lie in front of the mouth (now called prosthomeres), as well as of one or more somites behind it (opisthomeres). The first of the post-oral somites invariably has its parapodia modified so as to form a pair of hemignaths (mandibles). About 1870 the question arose for discussion whether the somites in front of the mouth are to be considered as derived from the prostomium of a Chaetopod-like ancestor. Milne-Edwards and Huxley had satisfied themselves with discussing and establishing, according to the data at their command, the number of somites in the Arthropod head, but had not considered the question of the nature of the prae-oral somites. Lankester (2) was the first to suggest that (as is actually the fact in the Nauplius larva of the Crustacea) the prae-oral somites or prosthomeres and their appendages 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 contrary, this—viz that all the Arthropoda are to be traced to a common ancestor resembling a Chaetopod worm, but differing from it in having lost its chaetae 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.
| Fig. 4.—Diagram of the head and adjacent region of Peripatus. Monoprosthomerous. |
| m, Mouth. I, Coelom of the first somite which carries the antennae and is in front of the mouth. II, Coelom of the second somite which carries the mandibles (hence deuterognathous). III and IV, Coelom of the third and fourth somites. FP, Rudimentary frontal processes perhaps representing the prostomial tentacles of Polychaeta. Ant, Antenna or tactile tentacle. Md, Mandible. Op, Oral-papilla. P, Protocerebrum or foremost cerebral mass belonging to the first somite. D, Deuterocerebrum, consisting of ganglion cells belonging to the second or mandibular somite. (After Goodrich.) |
| Fig. 5.—Diagram of the head and adjacent region of an Arachnid. Diprosthomerous in the adult condition, though embryologically the appendages of somite II and the somite itself are, as here drawn, not actually in front of the mouth. |
| E, Lateral eye. Ch Chelicera. m, Mouth. P, Protocerebrum, D, Deuterocerebrum. I, II III, IV. Coelom of the first, second, third and fourth somites. (After Goodrich.) |
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. Penpatus, the type-genus of the class Onychophora, stands at the base of the series with only a single prosthomere (fig. 4). In Peripatus the prostomium of the Chaetopod-like 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. 4); its parapodia have horny jaws on their ends, like the claws on the following legs (fig. 9), 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, prosthomerp 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 protocerebrum, and may also be concerned in the history of the very peculiar paired eyes of Peripatus, which are like those of Chaetopods in structure—viz vesicles with an intravesicular lens, whereas the eyes of all other Arthropods have essentially another structure, being “cups” of the epidermis, in which a knob-like or rod-like thickening of the cuticle is fitted as refractive medium.
In Diplopoda (Julus, &c.) the results of embryological study point to a composition of the front part of the head exactly similar to that which we find in Onychophora. They are deuterognathous.
The Arachnida present the first stage of progress. Here embryology shows that there are two prosthomeres (fig. 5), 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. 5), 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 altogether aborted. The appendages of the second prosthomere are the well-known chelicerae of the Arachnids, rarely, if ever, antenniform, but modified as “retroverts” or clasp-knife tangs in spiders.
The Crustacea (fig. 6) and the Hexapoda (fig. 7) agree in having three somites in front of the mouth, and it is probable, though not ascertained, that the Chilopoda (Scolopendra, &c.) are in the same case. The three prosthomeres or prae-oral somites of 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 the pair of appendages has not a merely tactile jointed ramus, but is converted into a claw or clasper. Three neuromeres—a proto-, deutero-, and trito-cerebrum—corresponding to those three prosthomeres are sharply marked in the embryo. The fourth somite is that in which the mouth now opens, and which accordingly has its appendages converted into hemignathous mandibles. The Crustacea are tetartognathous.