This apparatus appears to us to correspond, by its position, to that which is termed, in in sects, the salivary apparatus, and in silk-worms the silk-glands : it is even possible that the poisonous fluid itself, mingling with the ani mal juices which the spider introduces by suction into its stomach, serves to facilitate digestion.
Spiders are not the only animals of their class that are provided with this kind of organs. Scorpions have also a poison-apparatus, but in a very different position. It is not placed in the mandibles, but at the posterior part of the body, in the last segment of the tail-like abdo men. Every one is familiar with that pyriform dilatation which the scorpions carry at the end of the tail ; it is terminated by a little sharp hook generally curved backwards. Near its termination there may be observed, as in the mandibulous hook of spiders, a very minute orifice, or, according to some authors, two dis tinct fissures. It is from this part that a lim pid fluid, having strongly-marked poisonous qualities, exudes; and, corresponding to the fo ramen within, there is the neck of a little blad der which is the true secretory organ. Little is known respecting its structure : according to the observations of Treviranus it is surrounded by a horny substance and provided with a muscle, which most probably has for its func tion the compression of the vesicle, and the consequent expulsion of the poison.
Apparatus jrO secreting the fluid which con cretes in the air.--This apparatus is peculiar to Certain arachnidans : it does not exist in the scorpions nor in many other genera ; but when present it is always situated at the posterior part of the body. The threads by which the spiders suspend themselves, and of which they spin their webs, are emitted from the extre mity of the abdomen. There we find, in the vicinity of the anal aperture, several small appendages, which it is important not to con found with one another, (fig. 96.) Of these there are two which are small articulated hairy and filiform processes (b b ;)'" the others are spinnarets, or the or gans by which the silky threads are emit ted. Of the latter, four may generally be counted, (c d.) Their
structure is very re markable; it has been described by many anatomists, and among others by Lyonnet in his posthumous Memoirs. This patient anato mist has discovered that the surface of each of the spinnarets is pierced by an infinite number of minute holes, from each of which there escapes as many little drops df a liquid, which, becoming dry the moment it is in contact with the air, forms so many delicate threads. Im mediately after the filaments have passed out of the pores of the spinnaret, they unite first together, and then with those of the neighbouring spinnarets to form a common thread ; so that the thread of the spider, as it is employed in the manufacture of the web, or such as the creature suspends itself by when hanging from one's finger, is composed of an immense number of minute filaments, perhaps many thousands, of such extreme tenuity that the eye cannot detect them, until they are all twisted together into the working thread. Lyonnet has made a still more curious observation : he detected in Tege naria civilis (Walck.) a different anatomical structure of the four spinnarets. The pair which is above and a little longer than the other, presents on its surface a multitude of small perforations, (fig. 97,) the edges of which do not project, and which, therefore, resemble a sieve. This structure has also been well descri bed by Leuwenhoeck, Roe sel, Treviranus, &c.
The other pair,shorter and lower than the preceding, differs still further by having pro jecting or mamillary tubes independent of the perforations which also exist and are analo gous to those above described. The tubes are hollow,and perforated at the extremity (fig. 98, a). Lyonnet supposes that agglutinating threads issue from these tubes, while those which are emitted from the per forations do not poss ess that property. We may observe, indeed, upon throwing a lit tle dust on a spi der's web, such as the circular one of aranea diadems, that it ad heres to the threads which are spirally posed, but not to those that radiate from the -_, ircum entre to the cference; the latter are also _ circumference; stronger than the others.