Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/293

DESCRIPTION.] ABACHNIDA 273 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) respira tion 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 ali mentary 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 Aca ridea, the above canal has large 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 abdomen 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 (gen erally 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 dis tinct and separate sexes, the single exception known being the Tardigrada, a group of Acaridea approaching nearly to the Eutozoa, 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 exter nal 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 scorpions) 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 tho 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 organised forms. The above being a general summary of the external and internal organisation 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, form ing structural complications of several marked kinds, whence we get the different orders, being the next subor dinate 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 organised, we begin with the Acaridea, comprising the mites and ticks, and including also the Tardiyradidcs, 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 Solpvgidca, a small but very distinct group, constituting a link in one plane between the Pkalangidea 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, Araneidat, or true Spiders. Orders. I. Acaridea. II. Pycnogonidea. Sub-Class. AP.ACHNIDA. I III. Phalangidea. IV. Solpugidea. V. 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. PYCXOGOXIDEA. 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 Ions, and very slender ; the two

H. - 35