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.
It is in Spiders that the concentration of the nervous system reaches its climax; for in them we find the whole series of ganglia (encephalic, thoracic, and abdominal) aggregated together, and fused, as it were, into one great central brain, from whence nerves radiate to all parts of the body. The extent to which centralization is here carried will be at once appreciated by reference to the annexed figure (fig. 190): the encephalic masses (a a), whence the optic nerves distributed to the ocelli are derived, are in close contact with the anterior part of a large ganglion (c), that represents all the abdominal ganglia collected into one mass; and from the posterior part of this, nerves (n n), destined to supply the parts contained in the abdomen, derive their origin. The thoracic ganglia (ee) are fusiform, and placed on each side of the mass (c), with which they are apparently amalgamated at one extremity, while from the opposite they give off the nerves appropriated to the legs.
Fig. 190. Nervous system of the Spicier: a a, encephalic ganglia, from which are derived the optic nerves; e e, thoracic ganglia, forming by their coalescence the central mass (c); n n, nerves supplying the abdomen.
(975). The ocelli or eyes of Arachnidans have been minutely investigated by Muller*, and seem to present a type of structure very far superior to that of insects. In the Scorpion, this distinguished anatomist succeeded in detecting most of the parts which enter into the construction of the eye of a vertebrate animal, and, moreover, a great similarity in their arrangement. The cornea, a globular lens, the aqueous and vitreous humours, the retina and choroid, were all found nearly in their usual relative positions; so that the sense of vision in these animals must be extremely perfect.
(976). The sexual organs of the male and female Arachnidans exhibit very great simplicity in their structure. The testes, or secreting vessels, of the male Spider are two long caeca (fig. 191, b), lodged in the abdomen, and terminating by simple orifices at the ventral surface. No external intromittent organ is perceptible; and it was on this account that the peculiar apparatus above referred to, situated at the extremity of the maxillary palpus, was so long considered as giving passage to the impregnating secretion. The singular instrument already described (§ 957) would seem, indeed, to be in some manner really subservient to the fecundating process, being used most probably as an exciting-agent preparatory to the intercourse between the sexes.
(977). The ovigerous system of the female is equally devoid of complication, and, like the male testes, consists of two elongated membranous sacculi, in which the eggs are formed and brought to maturity. The impregnation of the ova is evidently effected by the simple juxtaposition of the external orifices of the two sexes: yet such is the ferocity of the female spider, that the accomplishment of this is by no means without risk to her paramour; for the former, being far superior to the male both in size and strength (fig. 192, a, b), would infallibly devour him, either before or after the consummation of his purpose, did he not exercise the most guarded caution and circumspection in making his advances.
Fig. 191. Generative organs of male Spider: a a, pul-monibranchiae; b, testes; o, cephalothorax.
* Ann. des Sci. Nat. torn. xvii.
Fig. 192. A. Female Spider. B. Male of the same species. C. Arrangement of the eyes.
(978). One peculiar characteristic of the Araneidae is the possession of a spinning apparatus, whereby the threads composing their web are manufactured. The instruments employed for this purpose are situated near the posterior extremity of the abdomen (fig. 194a, h), and consist externally of four spinnerets and two palpiform organs (fig. 193, a, b.) Each spinneret, when highly magnified, is found to be perforated at its extremity by innumerable orifices of extreme minuteness (fig. 193, c), through which the filaments are drawn; so that, unlike the silk of the Caterpillar, the thread of the Spider, delicate as it is, is composed of hundreds of smaller cords, sometimes woven together by zigzag lines, and thus exhibiting a structure of exquisite and most elaborate composition. The fluid silk, which, when it is drawn through the microscopic apertures of the spinneret, affords the material whereof the web is constructed, is secreted in a set of glands represented in the subjoined engraving (fig. 194.) The secerning extremities of the glandular tubes are composed of branched caeca (s), whence arise long and tortuous ducts (a a a), that become dilated in their course into reservoirs for the secreted fluid, and terminate by several canals at the base of the external spinning tubuli. Various are the purposes to which the different species of the Araneidae convert the delicate threads thus produced. Some construct silken tubes or cells in which to conceal themselves from pursuit, and from this retreat they issue to hunt for prey in the vicinity of their abode; others strew their filaments about at random, apparently to entangle passing insects; many make nets composed of regular meshes, and spread them out in favourable situations to entrap their victims (fig. 192); while a few species, enveloping their eggs in bags of curious construction, carry them about attached to their bodies, and defend them with the utmost courage and pertinacity: even in water these webs are turned to many singular uses; and ropes, nets, and even diving-bells are at the disposal of aquatic species furnished with this extraordinary spinning machinery. (979.) A few only of the most remarkable applications of this delicate material can be noticed in this place. The Mason-spiders (Mygale) excavate for themselves subterranean caverns, in which these marauders lurk secure from detection even by the most watchful foe; nor could any robber's den which ever existed in the wild regions of romance boast more sure concealment from pursuit, or immunity from observation. The construction of these singular abodes has long excited the admiration of the naturalist: a deep pit is first dug by the Spider, often to the depth of 1 or 2 feet, which, being carefully lined throughout with silken tapestry, affords a warm and ample lodging; the entrance to this excavation is carefully guarded by a lid or door, which moves upon a hinge, and accurately closes the mouth of the pit.