This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(949). Trombidium is the only Acaridan in which either M. Dujardin or Treviranus could discover the presence of a tubular two-branched ovarium; generally speaking, throughout this order of Arachnidans, the ova are produced in the substance of the general tissue of the body, without the presence of any ovarian apparatus with distinct parietes being apparent. The genus Oribates produces living embryos covered with a soft and wrinkled integument, which, as its development advances, becomes hard and crustaceous; in these Acari, therefore, in order to enable them to bring forth their young, it is necessary that the orifice of the vulva shall be of extraordinary dimensions; and accordingly it is found to be, in species thus constituted, a large oval orifice, occupying one-third or one-fourth of the entire length of the body, the opening being closed by two valves. In front of this large orifice, which is placed posteriorly, is another round opening, likewise terminated by valves, which gives issue to a long membranous tube, folded longitudinally and furnished with retractor muscles.
It would appear possible, therefore, that this is a penis, and that Oribates is hermaphrodite; for, seeing that the young are born alive, it cannot be looked upon as an ovipositor, or as furnishing any secreted defence for the ova.
(950). In conclusion, it seems evident, from the above-recorded circumstances connected with the organization of the Acaridan Arach-nidans, that much of their history is as yet involved in great obscurity; sufficient, however, has been said to show that much interest is attached to many points in the economy of these minute creatures.
(951). The rest of the Arachnidans breathe by means of lungs, or, as they are more properly designated, pulmonary branchice, and consequently, in contradistinction to the last-mentioned, are called by zoologists Arachnida Pulmonaria; such are the Scorpions and Spiders.
Fig. 184. Scorpion (ventral aspect).
(952). The Pedipalpi, forming the second division, are at once recognized by the peculiarity of their external configuration. Their palpi (the representatives apparently of the maxillary palpi of Insects) are exceedingly strong, and furnished at their extremity with a prehensile forceps; the hinder part of the body, corresponding with the abdomen of Insects, is much prolonged, and composed of numerous articulated segments, terminated in the Scorpion tribe by a sharp unciform sting (fig. 184) armed with a venomous secretion.
(953). The third section embraces the Araneidae, or Spiders, distinguished by having the abdomen short and globular, and furnished, moreover, near its posterior termination with spinnerets, by means of which these animals manufacture silken filaments applicable to a great number of purposes, and especially employed in constructing what is usually named the spider's web. The maxillary palpi in the females are simple, and more or less resemble feet; but in the males they often form a remarkable apparatus, to be described in another place. The jaws are also armed with sharp and hooked fangs, and perforated near their points for the emission of a poisonous secretion provided for the destruction of their prey, the venomous properties of which emulate that of the most formidable Serpents, and in like manner speedily terminate the sufferings of the victim.
(954). Beginning with the first division, we shall now proceed to place before the reader such facts as have been ascertained, connected with the anatomical structure of the class under consideration. In the Aearidce, or Mites, the skin of the entire body is so soft, that any annulose structure is scarcely distinguishable; the division, however, into cephalothorax and abdomen is sufficiently evident. The eyes are minute black points, never exceeding four in number, and resembling the ocelli of insects. Eight feeble legs are articulated with the thorax, properly so called. The mouth seems adapted to suction; and the jaws form a piercing instrument barbed at the extremity. The structure of the respiratory stigmata or spiracles would seem to differ very considerably from those of insects. According to Audouin, in the species which he examined (Ixodes Erinacei*), each spiracle resembles a spherical tubercle perforated by an infinite number of small holes, in the centre of which may be remarked a larger circular plate; and it is through these numerous foramina that the air enters the body and gets into the trachea?.
(955). The Pulmonary Arachnidans, both of the pedipalp and spinning divisions, are strictly carnivorous in their habits, living upon the juices of the insects they destroy; and we may consequently expect, in the construction of their alimentary apparatus, a simplicity proportioned to the facility with which highly nutritive food composed of already-ani-malized materials is capable of being assimilated. The mouth varies somewhat in its conformation; and if we compare the pieces composing it with those we have found mandibulate insects to possess, we shall have good reason for surprise in noticing the strange uses to which some parts of the oral apparatus are converted. In Scorpions (fig. 184), the apparent representatives of the mandibles of an insect are transformed into a pair of small forceps, each being provided with a moveable claw; these, therefore, of themselves form prehensile organs adapted to seize prey and hold it in contact with the mouth. But it is in the maxillce that we find the most extraordinary metamorphosis; for the maxillary palpi, so small in insects, are found to be developed to such prodigious dimensions, that they far surpass in size and strength any of the ambulatory extremities, and, from their resemblance to the claws of Crustaceans, have given the character from which the name of the division is derived 1. Each of these formidable organs is terminated by a strong pair of pincers, and thus the maxillary palpi become conthe young are bom struments either for attack or defence. The repre-as furnishing labium of an insect, in the Arachnidans, has no palpi (950) with it.