Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/292

Rh AKACHNIDA [GENERAL In tin s diagram the Myriapoda are placed, in the position ordinarily assigned to them, between the Insects and Arachnids ; but perhaps a more natural position would be after the Crustacea, for they appear to connect the Insecta and Crustacea more than they do the Insecta and Arach- nida. The point, however, of the special position of any large group in a linear series is tolerably unimportant. A linear arrangement is of course the only one possible on paper, but it cannot express the numerous cross relationships that become evident when the affinities of special groups iire closely studied. By placing the Myriapoda after the Crustaceans, we seem to get an ascending series in respect to the organs of locomotion : in the Insecta, six always attached to the thoracic segments ; in the Arachnida, eight almost always attached to the thoracic segments ; in the Crustacea, eight, ten, and upwards frequently attached to the abdominal as well as to the thoracic seg ments ; in the Myriapoda, twenty-four and more always attached to the abdominal as well as other segments. The relation of the Myriapoda to the Crustacea through the attachment of limbs to the abdominal segments is thus expressed ; and by the meeting of the two ends of the line, the always acknowledged affinity of the former to the Insecta is also expressed. Having thus briefly explained the most obvious relation ships of the Arachnida, let us now proceed to a more detailed, though still general, diagnosis of this group. And first, in respect to their external organisation, Arachnids are articulate animals, 1 with eight articulated legs, each in general consisting of seven joints ; the head and thorax are soldered together into one piece (cephalo-thorax), from which with few exceptions the legs invariably spring. In one group of the Arachnida (Holpugidea] the cephalo- thorax is separately segmented, as it is also in one family of the Thelyphonidea (Tartarides) ; the rest of the body forms also one more or less homogeneous piece (abdomen). The abdomen is always more or less closely attached to the cephalo-thorax ; in some instances it is soldered to the thorax, and forms with it one undivided piece, in other cases it is only joined to it by a narrow pedicle. At times, also, the abdomen constitutes a simple unsegmented portion, at other times a segmented or annulate portion of the body, occasionally, as in the true scorpions, prolonged into a .segmented tail : in one small but very distinct group {Thelyphonidea), the abdomen is prolonged into either a simple button, a short jointed, or a more or less long setiform tail; in another large group (Araneidea) the abdo men terminates with organs (spinners and spinnerets) for spinning threads. The eyes (when present) are simple, always sessile, and placed on the fore part of the cephalo- thorax; they seem to represent the simple ocelli of the Insecta ; the large compound eyes of that order having here no representation, the caput of Arachnids appearing to begin at a point posterior to that which bears the antennae and compound eyes of insects. In Arachnida the number of eyes varies from two to twelve. In front of and articulated beneath the fore part of the cephalo-thorax, and moving in different planes in different groups, are two independent, variously modified, organs for seizing and compressing the insects or other substances on which Arachnids in general prey ; these organs are often called mandibles, but more generally, and very appropriately, falces : these are considered by some systematists to be the true homologues of the antennae 2 in insects, and to have 1 Embryology shows that the first pair of legs in arachnids are homologous to the labial palpi of insects (Claparede, I. c. post). This is strictly true according to their use in one order Thelyphonidea ; and in many of ti.e Araneidea also these legs are chiefly used for feel ing and exploring the obstacles in their way. 2 Hence their name among French arachnologists, &quot; Antenues- Pinces.&quot; been derived from those portions of the insect organisation by long and unceasing modification ; when, however, the caput of an Arachnid (scorpion or spider, for instance) is compared with that of a coleopterous or hymenopterous insect, in which the mandibles are well developed, there seems far more reason to conclude that the antennae of the insect have no homoloyue at all in the Arachnid (Claparede, I.e. 2)st), but that the falces of the latter are the true representatives of the mandibles of the former. Behind the falces (and also used in raanducation) are two other large movable portions of structure called maxillce; these vary in shape and size, and form in fact lateral and hinder boundaries to the mouth, as well as an apparatus for com minuting and squeezing the food substances ; from each of these maxillae, on the outer side, springs a palpus of four or five joints, varying in structure and use. Between the maxilla} most Arachnids have also a fixed piece (labinm) of various form, completing the hinder limits of the mouth organs, and in some (perhaps in most) there is, within the parts already named, another portion (tongue), not yet suffi ciently observed either in regard to its form or use, but probably acting so as to hinder choking. In some Arachni- dans these different parts of the mouth are soldered together and form a tolerably simple sucking apparatus, analogous to the mouth of some insects (Hemiptera, &c.) As observed above, Arachnids are not, in a proper sense, subject to meta morphosis : in most of them there is little real change after they leave the egg; though, in some of the lowest forms, in deed, there are after-changes by moiiltings of the skin, which approach the incomplete metamorphoses of some insects. With respect to the internal organisation of the Arach nida, it might perhaps be enough here to refer to the more detailed accounts given further on, where the separate orders are under consideration ; but the following summary will probably make our general view of the whole sub-class more complete. Muscular System. Of this it is enough to say that it is similar to that of the rest of the Articulata, consisting of flexor and extensor muscles, situated within the hollows of the limbs, besides groups of fibres by which the epider mis and parts within it are connected and held together. On the outer surface these groups frequently show, at their points of union, boss-like marks or fovese, or im pressed spots of various forms and sizes, often presenting by their position distinctive peculiarities of form ; it is probable that many others of the external specific. markings are dependent on the course and position of the muscular fibres, as well as of the heart and other organs. Circulator)j System. The vital fluid is circulated by means of an elongated muscular vessel or heart, varying in form, and, instead of being placed, as in the Vertebrata, on the ventral side of the body, extending along the back of the ab domen. This vessel is often divided into chambers 3 or com partments by valves, having also valvular orifices on its upper side for the flowing in of the fluid (Newport), and giving off vessels (arteries) for its distribution to the rest of the body. In the lower forms, however, of Arachnids (among the Aca- ridca) no such principal muscular vessel is found, the vital fluid being in such cases supposed to circulate generally in the body, and to be distributed irregularly into different portions by the muscular movements of the intestinal canal. Respiratory System. Arachnids breathe by means of tracheae (spiracular tubes, as in the Insecta), as well as by pulmo-branchwe, said to be a kind of compound of the gill of fish and the lung of mammals, though in reality there seems better reason, as we shall see further on (p. 293), to consider the pulmo-branchise as merely peculiarly modified tracheae; 3 L. Dufour (Savants etranyers, xiv. pp. 594-609) controverts this in respect to the Scorpionides, and apparently with success, establish ing the fact of the dorsal vessel being simple and unchambered.