Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/294

Rh 274 AKACHNIDA [ACARIDS. 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 ; falees one-jointed, didactyle ; palpi terminating with a capsule containing a peculiar organ, the use of 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&quot; 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; falees 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, distri buted 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); falees monodactylous ; legs of the first pair exces sively 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-jointed tail, or a multi-articulate setiform one; respiration pulmo-brauchial; 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 spin ning; falees 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, some times 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, xxpon the families and genera comprised in them, adding a few general remarks upon each group. Order I. ACARIDEA. The inclusion in this order of such aberrant forms as Demodex, Pentastoma, and Tardigrada, somewhat destroys its homogeneity; undoubtedly they indicate a passage from the Annelides (Anarthropoda) to the Arthropoda, but whether it would not be more consistent with a strict scientific method to separate the two latter, at least, from the Acaridea, and make each the type of a distinct order, is a point for the future consideration of systematists. At present the balance of opinion appears to be for includ ing them as the lowest transitional forms of the lowest order of Arachnids. With regard to the position of Tardi grada, see Packard s Guide to the Study of Insects, p. 6G8, where Claparede is quoted from his /Studien an Acariden ; also Introduction to the Classification of Animals, Huxley, pp. 123-124. where the position of Pentastomides is also considered. See likewise on this, Cobbold s Entozoa, part iii. pp. 393-402, where reference is made to Leuckart s Bau- und Entwicldunysgcschichte der Pentastomiden. Owing to the excessively small size of most of the Aca ridea, and their obscure mode of existence, much of their economy and internal structure is yet unknown. The follow ing is an endeavour to present their characters in somewhat greater detail than in the short abstract given above. EXTERNAL STRUCTURE. In their general form the Acaridea are more or less round, oblong, or oval; the integu ment in some is soft, in others coriaceous, and some are quite hard, like a Coleopterous insect. The cephalo-thorax and abdomen are consolidated into one piece devoid of articulations ; not only is the head, as in other Arachnids, soldered to the thorax, but the portion (cephalo-thorax) formed by this union is generally joined invisibly to the abdomen, which is consequently merged in the general covering of the body. In the family Bdellides, however, and also in some other instances, the junctional points of the different parts of the body are visible enough. The legs are eight in number in the mature acarid, except in Pentastoma, which in a state of maturity has none (fig. 1) the four abnormal-looking legs visible in the young state disappearing in the adult (fig. 2). Many other Acaridea have six legs only until the last moult, when maturity is attained, one or two, even in the adult state, being said to have but four. Each leg consists in general of seven joints, and the tarsi end, in some, with two movable hooks ; in others the tarsus is dilated at its extremity as if for feeling with, while in some of those whose habits are aquatic it is expanded; in short, the structure of this part is very various, and adapted to the habits of the different genera. 1 Duges distinguishes the different kinds of legs as follows : 1. Palpatorii ; 2. Gressorii ; 3. Remigantes ; 4. Cursorii ; 5. Textorii ; 6. Carunculati. The parts of the mouth consist of two movable pieces, or falees, in front of which is another piece (labium) ; on each side of the labium is a strong piece (maxilla), and from the outer side of each of the maxillae springs a palpus of four or five joints. The different forms and structure of these parts give good characters for the subordinate divisions of the Acaridea. The mouth parts in the species of Acaridea present greater variety than is to be found in any other order, no doubt owing to the greater variation in their mode of existence. In some instances the falees, maxillse, and labium form, by their union, a sort of tube or pro boscis fitted for piercing, holding on to, and sucking the juices of their prey ; when not so united, the falees are terminated variously by a didactyle claw, somewhat like the claw of crab, or by a movable fang (sometimes two fold) as in the true spiders (Araneidea), or they consist of &quot; two long styles, which by moving backwards and forwards alternately, perforate the substance of their prey.&quot; The palpi of Acaridians are also variously formed, and, like the legs, have been minutely and well distinguished by Duges, who (I.e.) divides them into seven kinds. 1. Rapaces, armed with hooks for seizing hold with. 2. 1 Duges s &quot;Rccherches sur 1 ordre dcs Acariens,&quot;.4n?i. Sc. Nat., 2 e ser. t. i. ed. 2.