CLASS AKACHNIDA. 37 



which are often spoken of as jaws, but which really corre- 

 spond with the feelers ("antennae") of insects. These sin- 

 gular organs (fig. 15, B, n) are in the form of two powerful 

 jaw-like structures which terminate in strong curved hooks, 

 and are movably jointed to the head. The hooks are per- 

 forated by a minute aperture at the point, communicating 

 with a poison-gland ; so that when one of these fangs is 

 struck into the body of an insect, a drop of a poisonous 

 fluid is forced out into the wound. In this way the spider 

 kills the small animals upon which it feeds. 



The mouth is closed behind by a plate representing the 

 lower lip (fig. 15, C, l), and has at its sides two genuine jaws 

 {m), which carry the jointed feelers already alluded to. ' 



The front portion of the body of the Spider, as before 

 remarked, is truly the amalgamated head and thorax. 

 Behind this, and united with it by a narrow stalk, is the 

 egg-shaped, hairy mass, which constitutes the hinder half 

 of the body (fig. 15, A, a). This, though not exhibiting 

 any distinct segments, is really the "abdomen," and corre- 

 sponds with the "tail" of the Lobster. It does not support 

 any legs or appendages, but at its hinder end are situated 

 three pairs of minute conical eminences, which spin the 

 fibres which compose the webj and which are termed the 

 " spinnerets." The substance which composes the fibres 

 out of which the web is constructed, is secreted within the 

 body in a fluid form by certain special glands. The fluid 

 silk is then cast into its proper thread-like form by being 

 passed through the spinnerets, which are perforated by 

 numerous very minute pores or holes. In this way the 

 silken thread of the web is really composed of numerous 

 very delicate filaments woven together. 



Upon the under surface of the abdomen, far forwards, 

 are situated two small openings which communicate with 

 the breathing-organs. These latter have the form of little 

 sacs or air-chambers, the lining membrane of which is 

 thrown into numerous folds like the leaves of a book (fig. 

 15, D), and is richly supplied with blood. The air is 

 admitted directly to these sacs, and the blood is thus 

 purified ; so that the Spider is an air-breathing animal. 



