catory appendages. In the higher Arachnida, the mouth is provided from before backwards with the following appendages (figs. 144, 145): 1. A pair of "falces," or "mandibles," used for prehension; 2. A pair of "maxillæ," each of which is provided with a long jointed appendage, the "maxillary palp;" 3. A lower lip, or "labium." In the Scorpion, an upper lip, or "labrum," is also present.* In the Spiders (fig. 145, 4) each falx or mandible terminates in a sharp movable hook, which possesses an aperture at its extremity communicating by a canal with a gland, which is placed in the preceding joint of the mandible, and secretes a poisonous fluid. The maxillary palps in the Spiders are long, jointed appendages, terminated in the females by pointed claws, Fig. 145.-Morphology of Arachnida. 1. Organs of the mouth in the Scorpion, on one side: m Mandibles (antennæ) converted into chele, and called the cheliceræ ; Maxillary palpi greatly developed, and forming strong chelæ. 2. Telson of the Scorpion. 3. One of the abdominal segments of the Scorpion, showing the "stigmata, or apertures of the pulmonary sacs. 4. Tegenaria domestica, the common Spider (male), viewed from below: s Spinnerets; m Mandibles with their perforated hooks-below the mandibles are the maxilla, and between the bases of these is the labium; The maxillary palpi with their enlarged tumid extremities. but frequently swollen, and carrying a special sexual apparatus in the males. In the Scorpions (fig. 145, 1) the mandibles are short and The nomenclature ordinarily applied to the parts of the mouth in the Arachnida is a misleading one, so far as the homologies of this class with the Insecta are concerned. Thus the so-called "mandibles" are really the antenna; the "mandibles" themselves are absent, but the "chele" of the Scorpions may really represent the “mandibular palpi;" whilst the first pair of legs really corresponds with the "labial palpi," and the second pair of legs may possibly be a modification of a second pair of palps. terminate in strong pincers, or "chelicere." The maxillary palpi are also greatly developed, and constitute powerful grasping claws, or "chelæ." In the genus Galeodes, the mandibles, like those of the Scorpion, constitute "cheliceræ," though comparatively much larger and longer; but the maxillary palps are not developed into "chela." With regard to antennæ, these organs, as such, do not exist in the Arachnida. It is generally believed, however, that the mandibles of the Arachnida are truly homologues, not of the parts which bear the same name in the other Arthropoda, but of the antenna; and the name of "falces" is thus best applied to them. The antennæ, therefore, of the Spiders are converted into prehensile and offensive weapons; whilst in the Scorpions, as in the King-crabs, they are developed into nipping-claws, or chelæ. In the lower Arachnida, the organs of the mouth, though essentially the same as in the higher forms, are often enveloped in a sheath, formed by the labium and maxillæ, whilst the mandibles are often joined together so as to constitute a species of lancet. The mouth conducts by an oesophagus, sometimes by the intervention of a pharynx, to the stomach, which often carries longer or shorter cæca appended to it. The intestinal canal is short and straight, no convolutions intervening between the mouth and the anus. The terminal portion of the intestine is generally dilated into a cloaca, into which open, as a rule, branched or tortuous tubes, supposed to have a renal function, and to correspond with the "Malpighian vessels" of Insects. Salivary glands are generally present, and there is usually a well-developed liver. The circulation in the Arachnida is maintained by a dorsal heart, which is situated above the alimentary canal, and is wanting in the lower forms. Usually the heart is greatly elongated, and resembles the "dorsal vessel" of the Insecta. In the lower Arachnida, however, there is no central organ of the circulation, and there are no differentiated blood-vessels. All the Arachnida, except some of the lowest, breathe the air directly, and the respiratory function is performed by the general surface of the body (as in the lowest members of the class), or by ramified air-tubes, termed "trachea," or by distinct pulmonary chambers or sacs; or, lastly, by a combination of trachea and pulmonary vesicles. The "trachea" consist of ramified or fasciculated tubes, opening upon the surface of the body by distinct apertures, called "stigmata." The walls of the tube are generally prevented from collapsing by means of a chitinous fibre or filament, which is coiled up into a spiral, and is situated beneath their epithelial lining. The pulmonary sacs, or "tracheal lungs," are simple involutions of the integument, abundantly supplied with blood; the vascular surface thus formed being increased in area by the development of a number of close-set membranous lamellæ, or vascular plates, which project into the interior of the cavity. Like the trachea, the pulmonary sacs communicate with the exterior by minute apertures, or "stigmata" (fig. 145, 3), and they are to be regarded. as being simply greatly expanded tracheæ. The nervous system is of the normal articulate type, but is often much concentrated. Typically there is a cephalic or "cerebral" ganglion, a large thoracic ganglion, and often small abdominal ganglia. In some of the lower forms the articulate type of nervous system is lost, and there is merely a ganglionic mass situated in the abdomen. In none of the Arachnida are compound eyes present, and in none are the eyes supported upon foot-stalks. The organs of vision, when present, are in the form of from two to eight or more simple eyes, or ocelli." 66 In all the Arachnida, with the exception of the Tardigrada, the sexes are distinct. The great majority of the Arachnida are oviparous, and in most cases the larvæ are like the adult in all except in size. In some cases, however (Acarina), the larvæ have only six legs, and do not attain the proper four pairs of legs until after some moults. CHAPTER XXXVII. DIVISIONS OF THE ARACHNIDA. THE class of the Arachnida may be divided into the following orders : : ORDER I. PODOSOMATA (Pantopoda).- Respiration effected by the general surface of the body; limbs four pairs in number, elongated; abdomen rudimentary, unsegmented; sexes distinct. The members of this order, sometimes called "Sea-spiders," have been placed alternately amongst the Arachnida and the Crustacea, their true position being rendered doubtful by the fact that, though marine in their habits, they possess no differentiated respiratory organs. They possess, however, no more than four pairs of legs, and would therefore appear to be properly referable to the Arachnida. According to Dr Dohrn, however, the embryo is naupliiform, and this would support a reference of the order to the Crustacea. The commoner forms of the Podosomata (such as Nymphon and Pycnogonum) may be found on the sea-coast at low water, crawling about amongst marine plants or hiding beneath stones. Some species of the latter genus are parasitic upon fishes and other marine animals, but the common British species (P. littorale) is free when adult, and does not appear to be parasitic at any stage of its existence (fig. 147, a). The legs consist of four pairs, sometimes greatly exceeding the body in length, and containing cæcal prolongations of the digestive cavity for a portion of their length. The mouth is sometimes provided with a pair of 'cheliceræ," or chelate mandibles, and with two well-developed maxillary palpi, behind which in the female are a pair of false legs which carry the ova. The abdomen is rudimentary; but the cephalothorax is segment ed. Though there are no respiratory organs, there is a distinct heart. The sexes are in different individuals, and the larvæ have at first only two pairs of legs. A C Family 1. Pentastomida (Linguatulina).-The members of this family are worm-like parasites, which in their adult state are found in the interior of the frontal sinuses, the nose, or the lungs of the Dog, and of other Vertebrate animals. When fully grown (fig. 146) they are completely vermiform, with a soft annulated integument, and possessing no external organs except two pairs of retractile hooks, representing limbs, placed near the mouth. The adult thus presents an external resemblance to the Tania, from which, however, they are separated by the details of their internal organisation. There are no differentiated organs of Fig. 146.-A, Pentastoma tanioides, respiration or circulation, but the sexes are distinct. The larvæ (fig. 146, B) are found encysted in the liver or other internal organs of various Vertebrates (including man), and possess two pairs of articulated limbs. Family 2. Tardigrada (Macrobiotida or Arctisca). — This family comprises the so-called "Sloth " or " Bear Animalcules," which are microscopic animals found in damp moss and in the gutters of houses (fig. 148, B). In form, the body is some Fig. 147. Arachnida. a Pycnogonum littorale; b Tetranychus telarius, one of the "Sociable" mites; Hydrachna globulus, one of the "Water-mites." what vermiform, with four pairs of rudimentary legs. The mouth is suctorial, with rudimentary jaws or stilets. They exhibit no traces of respiratory or circulatory organs, and, unlike the other Arachnids, they have the sexes united in the same individual. 林 C Fig. 148.-A, Demodex folliculorum, greatly magnified; B, Emydium testudo, one of the Tardigrada, greatly magnified; C, Sarcoptes scabiei, the Itcrmite, greatly magnified. Family 3. Acarida.—This family includes the Mites, Ticks, and Water-mites, some of which are parasitic, whilst others are free, and some are even aquatic in their habits. The mouth |