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ARTHROPODA

appendages modified as maxillae. The fourth post-oral somite has its appendages converted into very large and powerful hemignaths, which are pi’ovided with poison-glands. The remaining somites carry single - clawed walking legs, a single pair to each somite. The body is anomomeristic, showing in different genera from 17 (inclusive of the anal and genital) to 175 somites behind that which bears the poison-jaws. No tagmata are developed. The genital ducts open on the penultimate somite. Trachere are developed which are dendriform and with spiral thickening of their lining. Their trunks open at paired stigmata placed laterally in each somite of the trunk or in alternate somites. Usually the tracheae open by paired stigmata placed upon the sides of a greater or less number of the somites, but never quite regularly on alternating somites. At most they are present on all the pedigerous somites excepting the first and the last. In Scutigera there are seven unpaired dorsal stigmata, each leading into a sac i whence a number of air-holding tubes project into the pericardial blood-sinus. Renal csecal tubes (Malpighian tubes) open into the proctodseum. Class 5.—Hexapoda. Head shown by its early development to be triprosthomerous and consequently tetartognathous. The first prosthomere has its appendages represented by the compound eyes and a protocerebrum, the second has the antennae for its appendages and a deutocerebral neuromere, the third has suffered suppression of its appendages (which corresponded to the second pair of antennae of Crustacea), but has a tritocerebrum and coelomic chamber. The mandibular somite bears a pair of gnathobasic hemignaths without rami or palps, and is followed by two jaw-bearing somites (maxillary and labial). This enumeration would give six somites in all to the head—three prosthomeres and three opisthomeres. Recent investigations (Folsom, 4) show the existence in the embryo of a prsemaxillary or supra - lingual somite which is suppressed during development. This gives seven somites to the Hexapod’s head, the tergites of which are fused to form a cephalic carapace or box. The number is significant, since it agrees with that found in Edriophthalmous Crustacea, and assigns the labium of the Hexapod to the same somite numerically as that which carries the labiumlike maxillipedes of those Crustacea. The somites following the head are strictly nomomeristic and nomotagmic. The first three form the thorax, the appendages of which are the walking legs, tipped with paired claws or ungues (compare the homoplastic claws of Scorpio and Peripatus). Eleven somites follow these, forming the abdominal “ tagma,” giving thus twenty-one somites in all (as in the higher Crustacea). The somites of the abdomen all may carry rudimentary appendages in the embryo, and some of the hinder somites may retain their appendages in a modified form in adult life. Terminal telescoping of the abdominal somites and excalation may occur in the adult, reducing the obvious abdominal somites to as few as eight. The genital apertures are median and placed far back in the series of somites, viz., the female on the seventh abdominal (seventeenth of the whole series) and the male on the ninth or ante-penultimate abdominal (nineteenth of the whole series). The appendages of the eighth and tenth abdominal somites are modified as gonapophyses. The eleventh abdominal segment is the telson, usually small and soft; it carries the anus. The Hexapoda are not only all confined to a very definite disposition of the somites, appendages, and apertures, as thus indicated, but in other characters also they present the specialization of a narrowly-limited highly-developed order of such a class as the Crustacea rather than a range from lower more generalized to higher more specialized forms such as that group and also the Arachnida present. It seems to be a legitimate conclusion that the most primitive Hexapoda were provided with wings, and that the term Pteiygota might be used as a synonym of Hexapoda. Many Hexapoda have lost either one pair or both pairs of wings ; cases are common of wingless genera allied to ordinary Pterygote genera. Some Hexapods which are very primitive in other respects happen to be also Apterous, but this cannot be held to prove that the possession of wings is not a primitive character of Hexapods (compare the case of the Struthious Birds). The wings of Hexapoda are lateral expansions of the terga of the second and third thoracic somites. They appear to be serial equivalents (homogenous meromes) of the tracheal gills, which develop in a like P°?^10n on the abdominal segments of some aquatic Hexapods. The Hexa,poda are all provided with a highly developed tracheal system, which presents considerable variation in regard to its stigmata or orifices of communication with the exterior. In some a serial arrangement of stigmata comparable to that observed in Ghilopoda is found. In other cases (some larvae) stigmata are absent; in other cases again a single stigma is developed, as in the smaller Arachnida and Chilopoda, in the median dorsal line or other unexpected position. When the facile tendency of Arthropoda to develop tracheal air-tubes is admitted, it becomes probable that the tracheae of Hexapods do not all belorm to one

original system, but may be accoimted for by new developments within the group. Whether the primitive tracheal system of Hexapoda was a closed one or open by serial stigmata in every somite remains at present doubtful, but the intimate relation of the system to the wings and tracheal gills cannot be overlooked. The lateral eyes of Hexapoda, like those of Crustacea, belong to the most specialized type of “compound eye,” found only in these two classes. Simple monomeniscous eyes are also present in many Hexapods. Renal excretory caeca (Malpighian tubes) are developed from the proctodaeum (not from mesenteron as in Scorpion and Amphipoda). Concluding Remarks on the Relationships to one another of the Classes of the Arthropoda.—Our general conclusion from a survey of the Arthropoda amounts to this, that whilst Peripatus, the Diplopoda, and the Arachnida represent terrestrial offshoots from successive lower grades of primitive aquatic Arthropoda which are extinct, the Crustacea alone present a fairly full series of representatives leading upwards from unspecialized forms. The latter were not very far removed from the aquatic ancestors (Trilobites) of the Arachnida, but differed essentially from them by the higher specialization of the head. We can gather no indication of the forefathers of the Hexapoda or of the Chilopoda less specialized than they are, whilst possessing the essential characteristics of these classes. Neither embryology nor palaeontology assists us in this direction. On the other hand, the facts that the Hexapoda and the Chilopoda have triprosthomerous heads, that the Hexapoda have the same total number of somites as the nomomeristic Crustacea, and the same number of opisthomeres in the head as the more terrestrial Crustacea, together with the same adaptation of the form of important appendages in corresponding somites, and that the compound eyes of both Crustacea and Hexapoda are extremely' specialized and elaborate in structure and identical in that structure, all lead to the suggestion that the Hexapoda, and with them, at no distant point, the Chilopoda, have branched off from the Crustacean main stem as specialized terrestrial lines of descent. And it seems probable that in the case of the Hexapoda, at any rate, the point of departure was subsequent to the attainment of the nomomeristic character presented by the higher grade of Crustacea. It is on the whole desirable to recognize such affinities in our schemes of classification. We may tabulate the facts as to head-structure in Cluetopoda and Arthropoda as follows :— Grade x (below the Arthropoda).—Agxatha, Afrosthomera. Without parapodial jaws; without the addition of originally post-oral somites to the prse-oral region, which is a simple prostomial lobe of the first somite ; the first somite is perforated by the mouth and its parapodia are not modified as jaws. = Ch^topoda. Grade 1 (of the Arthropoda).—Monognatha, MonoPROSTHOMERA. With a single pair of parapodial jaws carried by the somite which is perforated by the mouth; this is not the first somite, but the second. The first somite has become a prosthomere, and carries a pair of extensile antennse. = Onychofhora {Peripatus, <£-c.). Grade 2 (of the Arthropoda).—Dignatha, Monoprosthomera. The third somite as well as the second develops a pair of parapodial jaws; the first somite is a prosthomere carrying jointed antennse.
 * Diplopoda.

Grade 3 (of the Arthropoda).—Pantognatha, Diprosthomera. A gnathobase is developed (in the primitive stock) on every pair of post-oral appendages; two prosthomeres present, the second somite as well as the first having passed in front of the mouth, but only the second has appendages. = Arachnida. Grade 4 (of the Arthropoda).—Pantognatha, Triprosthomera. The original stock, like that of the last grade, has a gnathobase on every post-oral appendage, but three prosthomeres are now present, in consequence of the movement of the oral aperture from the third to the fourth somite. The lateral eyes are polymeniseous,