4 
INTRODUCTION. 
closely grouped in pairs, readily distinguished from each other by their relative positions. 
The pair situated nearest to the anus may be denominated the superior spinners; that furthest 
removed from the anus, the inferior spinners; and the mammulae placed between these 
extremes, the intermediate spinners; distinguishing them, when there are two pairs, by 
prefixing the terms superior and inferior. Fine, moveabffi papillae, differing in size in the same 
individuals, are situated at the extremity, or distributed along the inferior surface of their 
terminal joint, whence issues the viscous secretion of which the silken lines produced by 
spiders are formed. 1 
The superior pair of spinners, when triarticulate and considerably elongated, has been 
mistaken by Treviranus and other skilful zootomists for anal palpi. This singular error may 
be attributed to the peculiar disposition which the papillae, or spinning-tubes, connected with 
the superior mammulae, -when thus modified, commonly exhibit. Arranged along the under 
side of the terminal joint, they present the appearance of slender hairs dilated at the base; 
but if the spinners, when they are in operation, be carefully examined with a powerful 
magnifier, the function of the hair-like tubes may be ascertained without difficulty, as the fine 
lines of silk proceeding from them will be distinctly perceived. The purpose subserved by 
the superior mammulae, when very prominent and composed of several joints, is the binding 
down with transverse lines, distributed by means of an extensive lateral motion, the threads 
emitted from the inferior mammulae; by which process a compact tissue is speedily 
fabricated. 2 
On the under part of the abdomen, near its anterior extremity, there are two or four 
either slightly pubescent or naked plates of a whitish, yellowish, or brownish hue, having at 
their posterior margin a narrow, transverse orifice or stigma, communicating with the organs of 
respiration. In the space intermediate between these plates the external aperture of the sexual 
organs is situated in female spiders. It is fully developed in the adult state only. 
The internal organs of respiration in connexion with the anterior pair of stigmata present 
the appearance of membraneous sacs formed by lamellae applied to one another like the leaves 
of a book; each of these little receptacles opens into a common cavity, the membraneous 
margins of which adhere to a cartilaginous arc connected with the branchial operculum. 
These organs are common to all spiders, and, from the important function they perform, may 
be termed pulmonary branchiae. The posterior pair of stigmata occur much less frequently, 
and in communication with them there are numerous tracheae which radiate to the surrounding 
parts. 
An elongated, fusiform vessel, whose office is analogous to that of the heart in animals 
of a higher order, extends along the middle of the superior region of the abdomen, immediately 
beneath the integument, on which account it has been named the dorsal vessel. An artery 
is continued from both its extremities, and the colourless blood, which is returned to it by 
several veins on each side, is propelled forwards by the contraction of its muscular walls. 
The alimentary canal communicates with the mouth, between the maxillae, by a short, 
slender oesophagus of a delicate texture, terminating in the stomach, which is placed in the 
posterior part of the cephalo-thorax. The digestive tube then continues as a straight narrow 
1 ‘ Transactions of the Linnean Society/ vol. xviii, pp. 219-224. 
2 ‘ Researches in Zoology/ p. 298, et seq. 
