The nervous system of the Arachnids is ganglionic, consisting of nerve-knots (ganglia). In man and the larger animals, a ganglion is composed of two substances similar to the cortical and medullary sub stances of the brain, and differs from nerves in being firmer in texture, and covered with a membrane of closer tissue. In the Arachnida these nerve-knots are more concentrated, if the term may be used, than in Insects, and they are uniform in composition, rather than a chain of ganglions equally separated. Thus, in the Harvest-Spiders (Phalangia) there are a pair of nerve-knots in front of the gullet, and at the back of the gullet a medullary mass, apparently consisting of three ranges of nerve-knots united.
Wo know nothing of the organ of hearing in Arachnida, though it is certain enough that they do hear. Their eyes are all simple, not composite, like those of many insects. " The eyes of spiders and scorpions," says Swammerdam, " are externally formed exactly in the same manner, and are smooth, glittering, and without divisions ; and are as much dispersed as those that are disposed at random over the body. The Wolf Spider, which catches its prey by leaping on it, has its eyes placed in the same manner." In the greater number of Spiders they are S in number, but in some 6 (Dysdera and Segestria), and in others 2 (Phalangium). The arrangement of the eyes, when more than two, varies considerably in the different genera, and is taken advantage of in arranging them systematically, ou the principle first pointed out by Dr. Lister, and improved upon by Latreille, Leach, and Walekenser. Figures of various arrangements of the eyes iu spiders may be seen in ' Insect Miscellanies,' pp. 125, 126, after Audouin. (' Lib. of Ent. Knowledge') With regard to the sexes, male spiders are always much smaller than the females, being often not more than one-fourth the size. The feelers (palpi), also, in the male arc furnished with organs at the tip, which are of various forms, but usually bulging, whereas the feelers in the female gradually taper to a point.
Looking at the size of the female spider, and the eggs which she lays, it appears almost incomprehensible how they could be contained in so small a body. But, by observing them more closely, it may be discovered that they have not, like the eggs of birds, a hard ahell, but, on the contrary, are soft and compressible. Accordingly, before they are laid, they lie in the egg-bag (orarium) within the spider's body, squeezed together in a flat manner ; and only come into a globular form after they are laid, partly in consequence of the equal pressure of the air on every side, in the same way as we see dew-drops and globules of quicksilver formed from the same cause.
The eggs of spiders, it is worthy of remark, are in most cases, though not always, placed in a roundish ball ; • and, as there is nothing in nature without some good reason, if we can discover it, wo may infer that this form is designed to economise the materials of the silken web which the mother spins around them by way of protection. Whether wo aro right or not in this conjecture, there can be no question as to the manner in which the ball is shaped, as the writer has often observed the process. The mother spider, in such cases, uses her own body as a gauge to measure her work, in the same way as a bird uses its body to gauge the size and form of its neat. The spider first spreads a thin coating of silk as a founda tion, taking care to have this circular by turning round its body during the process. It then, in the same manner, spins a raised border round this till it takes the form of a cup, and at this stage of the work it begins to lay its eggs in the cup, not only filling it with these up to the brim, but piling them up above it into a rounded heap as high as the cup is deep. Here, then, is a cup full of eggs, the under half covered and protected by the silken sides of the cup, but the upper still bare, and exposed to the air and the cold. It is now the spider's task to cover these, and the process is similar to the pre ceding, that is, she weaves a thick web of silk all round them, and, instead of a cep-shaped nest like some birds, the whole eggs are inclosed in a ball much larger than the body of the spider that con structed it.
There is is singular mechanism for the purpose of placing the eggs in the proper position. The eggs, different from what takes place in birds, are excluded from a cavity just behind the breast. Here there is an organ placed somewhat in the form of a hook or a bent spatula, which the spider can move in such a manner as to direct every indi vidual egg which it lays to the exact spot in the nest-cup where it wishes it to be placed. The 'sense of touch in this organ must of course be very acute, as by touch it must he wholly guided ; for its eyes, though eight in number, and very piercing, are situated on the upper part of the head, and cannot be brought within sight of the nest.
The hatching of the eggs of one species (Epeira diadems) has been traced with great minuteness, and the successive evolution of the embryo figured with great skill, by M. Herold of Marburg.
M. Latreille, whose method has been generally followed both iu Britain and on the Continent, arranges the Arachaida into two orders, as follows : Class. Orders. Pulmonary sacs for respiration; six