158 REPORT—1858. 
The abdomen of spiders is covered by a tough integument, consisting of 
three layers: the external one is a thin transparent horny membrane, nearly 
colourless, but more or less densely covered with coloured hairs; beneath 
this lies a soft layer of pigmentary matter, upon which the peculiar colour of 
the body depends; for it may be observed, that, when the hairs with which 
the body of a spider is clothed are rubbed off, the integument beneath is 
usually of a dark tint. The third or inner layer consists of an expanded 
network of muscular fibres, which are irregularly interlaced, and which must 
enable the spider forcibly to compress the abdomen. The muscles forming 
this layer are very faintly, if at all, marked with transverse striz (see Plate XVI. 
fig. 1). 
At ohe apex of the abdomen, on the under side, is the anal tubercle, partly 
concealing the opening of the intestinal canal; and immediately in front of 
it are seated the spinnerets, a group of projecting processes or mammule; 
mostly articulated, and moveable in all directions. Their number is gene- 
rally six, but sometimes they are reduced to four, and, as Mr. Blackwall dis- 
covered, they amount to eight in one family. They are placed in pairs, 
closely grouped together. When six in number, the two anterior and two 
posterior are much larger than the two intermediate ones, which in a state 
of repose are hidden beneath the others (see Plate XVI. figs. 2and 3). The 
posterior spinnerets are often triarticulate, and have the terminal joints much 
prolonged and very hairy, when they have been called anal palpi, and supposed 
not to take any part in the construction of the webs; Mr. Blackwell, how- 
ever, demonstrated their true character, showing that they are provided with 
moveable papillz for the transmission of the silk, like the others. The ex- 
ternal anatomy of the spinnerets has been so fully described by Mr. Black- 
wall *, who has shown the number and arrangement of these papilla, which 
vary greatly in different species and on the different spinnerets, that I need 
not dwell further upon them. In Plate XVI. fig. 6, I have represented some 
of them, which are like hollow bristles with dilated bases. 
In the spiders belonging to the family of the Cinifloride, Blk. (the type 
of which is the common Ciniflo (Clubiona) atrox), there is a fourth pair of 
spinnerets. They are short, compressed, and inarticulate, and different in 
appearance from all the others. They are seated at the base, and in front 
of the ordinary anterior pair, and have each on the surface an oval flattened 
space perforated with an immense number of exceedingly minute pores, which 
are the orifices of the spinning tubes (see Plate XVI. fig. 5). 
The spinnerets are counected with the surrounding integument by means 
of diverging bands of muscular fibres (Plate XVI. fig. 3), which enable them 
to move in different directions ; these muscles are placed immediately beneath 
the skin, and their expanded extremities are inserted into it so that they are 
separated with it, unless dissected very carefully. 
In the interior of the abdomen, nearer to the base than the apex, and in 
female specimens opposite the opening of the oviduct, is a fixed spot, pro- 
bably tendinous in character, from which muscular bands radiate in various 
directions, keeping the different abdominal organs in their places. Some of 
these bands are inserted into the integument on both the ventral and dorsal 
surfaces of the body; others run backwards in straight parallel bundles, and 
pass into the interior of the different spinning mammule. The last-men- 
tioned fibres are strongly striated (Plate XVI. tig. ’7), while those passing from 
the same point to the skin, like the muscles fixing the margins of the 
spinnerets to the integument, are almost destitute of striz. When the abdo- 
* Report on the Araneidea, British Association.for 1844. . . 
