TICK.) Most of the existing orders of terrestrial Arachnida are repre sented in the Coal Measures. But those deposits have yielded re mains of genera referable to three distinct orders, the Haptopoda, Phalangiotarbi and Anthracomarti.
The Haptopoda in the wide confluence of the cephalothorax and abdomen, the apparent fusion of the abdominal segments and the pediform palpi superficially resemble some Opiliones ; but in the presence of r i abdominal segments, the structure of the ab dominal sterna, the separation of the basal segments of the pos toral appendages of the cephalothorax by a distinct sternal area and the tactile function of the legs of the first pair resemblance may be traced to some of the Caulogastra. One genus only, Plesiosiro, family Plesiosiromidae is known.
The Phalangiotarbi also recall the Opiliones in the above men tioned characters, but are distinguished from all the other orders by the progressive increase in length of the abdominal segments from the first to the ninth, and in the radial arrangements of the large wedged-shaped basal segments of the four pairs of legs round the small sternal area, those of the first pair concealing the basal segments of the other palpi; the genital sternal plate was situated between the last pair of legs. Families Phalangiotarbidae (Phalangiotarbus), Architarbidae (Architarbus, Gerapjirynus).
The Anthracomarti had a wide movable joint between the cephalothorax and abdomen, the abdomen consisting of as many as ten segments provided with pleural plates, those of the eighth overlapping small ninth and tenth segments; the pediform palpi and four pairs of legs were separated by a distinct sternal area very much as in the Araneae; but the structure of the abdomen is distinctive of the order. Families Brachypygidae (Brachypyge), Anthracomartidae (Anthracomartus), Anthracosironidae (An thracosiro), Eophrynidae (Eophrynus).
It is significant that although these extinct orders show gross resemblances to existing orders of epectinate Arachnida, serving to link more closely constituents of that section into a coherent assemblage, no fossil forms are known bridging in any way a structural interval between the Epectinata and the Pectinifera.
Certain other groups, namely the Pycnogoniea or Pantopoda, the Tardigrada and the Linguatulida or Pentastomida were for merly referred to the Arachnida. The last two seemed to have no claim to be so classified. For particulars concerning them see TARDIGRADA, and LINGUATULIDA. The Pycnogonida (q.v.), how ever, are like the Arachnida in some respects and were retained in the class by Lankester. But they are here regarded as a distinct class on account of several important characters they possess. There may be, potentially at all events, eight pairs of appendages set apart for locomotion, prehension or other purposes instead of the six pairs seen in the Nemomeristis Arachnida. Also the ab domen, always large and conspicuous in the Arachnida, as here understood, is reduced to a mere anal knob. More important still is the lodgement in the four parts of the body of the genital glands which open upon the basal segments of more or fewer of the ap pendages. These features in the organization of the Pycnogonida cannot be brought into agreement with that of the true Arachnida the orders of which form a coherent assemblage of more or less intergradational types of Arthropoda. (R. I. P.)