(3) ARACHNIDA - INTERNAL ORGANISATION
With respect to the internal organization of the Arachnida, 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 epidermis 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 foveae, or impressed 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.
Circulatory 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 Verbetrata, on the ventral side of the body, extending along the back of the abdomen. This vessel is often divided into chambers or compartment 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 Arachinds (among the Acaridea) 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 tracheoe (spiracular tubes, as in the Insecta), as well as by pulmo-branchioe, 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-branchiae as merely peculiarly modified tracheae; both these organs have external orifices (stigmata) variously situated, but commonly in some part of the abdomen. In some Arachnids (certain groups of the Araneidea) respiration is effected by both of the above organs; while in others (certain of the Acaridea) no distinct organs of respiration exist.
Digestive System - Digestion is effected by a simple alimentary tube or canal running from the mouth to the anus. This tube is various in its form, as well as in the intestines issuing from it; in some groups, as the Araneidea and Acaridea, the above canal has largely lateral caecal appendages; in others (Scorpionides and Thelyphonides) the alimentary canal has no such lateral enlargements. In some Arachnids there have also been found a liver (or mass of substance in the abodmne exercising the functions of a liver), an organ performing the part of kidneys, and salivary glands, often of large size, while in others no distinct organs of digestion exist.
Nervous System - This consists of ganglia or nerve-knots, formed by enlargements of longitudinal nervous cords (generally two in number), differing in position and in the nerves issuing from them. in one large group (Acaridea) where the abdomen usurps, as it were, the place of all the rest of the body, there is no longitudinal nerve cord; a single nerve mass occupies the interior of the abdomen, and sends forth nerves to the various surrounding parts.
Generative System. - Generation is effected by two distinct and separate sexes, the single exception known being the Tardigrada, a group of Acaridea approaching nearly to the Entozoa, and said to be hermaphrodite. In females the parts of generation consist of two sacs (ovaria), one on each side of the alimentary canal. These have a common external orifice (vulva), various in its shape, beneath the anterior extremity of the abdomen; and connected with this opening there is frequently an epigyne, or ovipositor, often of some length, and of characteristic form. In males (except among the phalangids and scorpirons) there is no intromittent organ, - the generative parts consisting of two long tubes, similarly situated to the ovaria in females, in which the seminal fluid is secreted; these tubes end in a simple external, very minute orifice, in a similar situation to the vulva of the female.
Arachnids are either oviparous or ovo-viviparous. As before observed, they undergo no proper metamorphosis, but as growth takes place there are more or fewer successive moultings of the skin; and complete development of all the parts does not occur until the last moult, when the creature becomes adult, and so fitted to perpetuate its kind.
With regard to the position held by Arachnids in the great scheme of nature, it seems, from the considerations shortly entered into above, undeniable that they are very closely linked to the rest of the Articulata by much, both of external and internal structure, common to the whole branch. The office they subserve is evidently the keeping down of the superabundant supply of the insect world; some even operating to check the too rapid advance of higher animals.
Assuming that the operation, in some shape or other, of evolution has brought the different groups of Articulata to the varied forms and conditions they now respectively present, it is plain that the higher, at least, of the arachnidous orders must, in all probability, have been modified from some one or more of the other groups, perhaps in the main from the Insecta and Myriapoda, - certainly not the Insecta, for instance, from the Arachnida. Thus, we should define Arachnids, generally, to be insects modified for the purpose of preying upon their congeners; while some of the lower forms of Arachnids, such as Demodex, Tardigrada, and Pentastoma, carry us back to the remote past, before the branch Articulata, in any of its great groups, had attained more than a very slight progress towards its present dignified and highly organized forms.
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Arachnida - Table of Contents