(4) THE ORDERS OF THE ARACHNIDA
The above being a general summary of the external and internal organization of the Arachnida, an examination of the whole group shows us that their different characters, and the ways in which their articulated plan of structure is carried out, are variously correlated and modified, forming structural complications of several marked kinds, whence we get the different orders, being the next subordinate groups, into which Arachnids are divided. These orders are seven in number, and form so many well-defined groups, but of very different extent. Advancing from the more simple to the more highly organized, we begin with the Acaridea, comprising the mites and ticks, and including also the Tardigradides, Pentastomides, as well as Demodex. The second order is that of the Pycnogonidea, marine parasites, which, in a certain plane, appear to connect the Acaridea with the next or third order, Phalangidea; these last are popularly known as "Harvest-men." Next to them, and forming the fourth order, come the Solpugidea, a small but very distinct group, constituting a link in one plane between the Phalangidea and Scorpionidea, which last, including the Chelifers, or False Scorpions, form the fifth order. Not far removed from these is the sixth order, Thelyphonidea, comprising groups which partake of the characters both of the Scorpionidea and of the next - the seventh and last - order, Araneidea, or true Spiders.
Orders:-- I. Acaridea. II. Pycnogonidea. III. Phalangidea IV. Scorpionidea. VI. Thelyphonidea. VII. Araneidea.
The following abstract shows the chief ordinal characters of these seven groups of Arachnids: -
Order I. ACARIDEA. - Cephalo-thorax and abdomen united so as to form one piece, generally without mark of union; palpi and falces variable in structure; the several parts of the mouth often united; legs terminating variously, generally eight in number, sometimes only six in the immature state, and in one or two instances but four, even when adult; eyes, when present, variable in number (2 to 6), and placed on the cephalo-thorax; respiration, when proper organs for it exist, wholly tracheal; nervous matter gathered into one large mass, or ganglion; reproduction ovo-viviparous as well as oviparous. Maturity is reached by quasi, or imperfect, metamorphoses in some of the groups.
Order II. PYCNOGONIDEA. - Cephalo-thorax segmented, linear; abdomen rudimentary, forming merely a small terminal segment to the cephalo-thorax; the mouth consisting of the termination of a tubular article forming the head, sometimes accompanied by some more or less distinct parts; legs eight, multi-articulate; in females a supernumerary pair, between the first two and used for bearing ova; organs of respiration, none; metamorphosis imperfect, as in the last order.
Order III. PHALANGIDEA. - Cephalo-thorax and abdomen of tolerably equal size, and united throughout their whole breadth, but the junctional line evidently marked; abdomen annulate or segmentate, caused by transverse folds in the epidermis; falces two-jointed, didactyle; legs in general inordinately long, and very slender; the two terminal joints multi- articulate, and ending with claws; eyes two, on the vertex of the cephalo-thorax; respiration tracheal; reproduction oviparous; no metamorphosis of even an imperfect kind.
Order IV. SOLPUGIDEA. - Cephalo-thorax distinct from the abdomen and segmented, the first segment forming the head; abdomen annulate; falces one-jointed, didactyle; palpi terminating with a capsule containing a peculiar organ, the use two which is not yet known; eyes two, on a small eminence at the fore part of the head in the medial line; respiration tracheal; legs terminating with two curved fingers; manner of progressive growth unknown.
Order V. SCORPIONIDEA. - Cephalo-thorax formed of one undivided piece, united to the abdomen throughout its entire breadth; abdomen annulate, prolonged (in one of the families) into a segmented tail, ending with a poison bulb armed with a sharp perforated point, through which the poison is emitted; falces didactylous; palpi terminating with a didactyle claw; eyes variable in number, two to twelve, variously grouped on the fore part of the cephalo-thorax; respiration, in one family, tracheal, in another, pulmo-branchial; ganglia several, distributed along the nervous chord; reproduction, in some oviparous, in others ovo-viviparous; organs for spinning beneath the fore part of the abdomen in one family; no metamorphosis.
Order VI. THELYPHONIDEA. - Cephalo-thorax undivided, but its segmental structure generally visible in grooves and furrows, following the course of the soldered joints; abdomen annulate, joined to the cephalo-thorax by a pedicle; palpi very strong, didactylous (in a modified form); falces monodactylous; kegs of the first pair excessively long, antenniform, the last joints very fine, multi-articulate, and devoid of a terminal claw; eyes, when present, eight, disposed in three groups on the fore part of the cephalo-thorax; abdomen either terminating with a button-like segment, a short two-joined tail, or a multi-articulate setiform one; respiration pulmo-bronchial; manner of progressive development unknown. In one family, ganglia two, closely united; in others unknown.
Order VII. ARANEIDEA. - Cephalo-thorax undivided, but traces of segmentation commonly more or less visible, united to the abdomen by a narrow pedicle; abdomen without segments or annuli, ending with organs for spinning; falces monodactylous; palpi more or less filiform in males, with the last joint more or less complicated in structure, and used in copulation, terminating in females (but not invariably) with a claw or claws, sometimes pectinated; legs terminating with two or three curved claws, and, except the third (which is rarely so), generally more or less pectinated; ganglia two or three in number; respiration tracheal, as well as pulmo-branchial; reproduction oviparous.
We have thus far obtained a general view of the leading groups of the sub-class Arachnida, both in respect to its relation to collateral groups of equal value, and the relation of its own members to each other, and also a synoptical view of the leading characteristics of the seven orders into which Arachnids may be divided. We now propose to touch shortly, but in greater detail, upon each of those orders, and, as far as our limits will permit, upon the families and genera comprised in them, adding a few general remarks upon each group.
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