ARACHNIDA. 257 



filled with aqueous humour and receiving a circular process of the 

 thick pigmental chorion which defines the pupil and confines the lens 

 to the anterior chamber. The pigment coats the retina, and covers 

 part of the optic nerve. 



Spiders have the sense of hearing, but neither the organ nor its 

 situation are known. The same may be said of the sense of smell. 

 The membrane lining the mouth and pharynx may have the faculty 

 of taste, and influence the Arachnidans in their choice of food. The 

 soft and often hairy integument must be to a certain degree sensitive, 

 but touch would appear to be exercised principally by the leg-like 

 instruments into which the maxillary and labial palpi are converted. 



The alimentary canal is short and straight in most Arachnidans : a 

 slight convolution of the intestine takes place in some spiders : that 

 of mites is straight and wide, but the stomach, in some, is produced 

 into lateral sacculi. In scorpions, the alimentary canal extends, 

 without any gastric dilatation or intestinal convolution, from the 

 mouth to the anus. Five short and straight diverticula are sent off 

 at equal distances from each side of the thoracic portion, and are 

 lost in the granular and seemingly adipose masses, which have been 

 regarded as a kind of epiploon by some, by others as an hepatic 

 organ. Two delicate capillary secretiog tubes unite on each side, 

 close to the intestine, and open into that part of the canal which is in 

 the anterior part of the tail-like abdomen. 



The spiders are remarkable for the minuteness of the pharynx and 

 oesophageal canal. Savigny believed that in some species there existed 

 three pharyngeal apertures, through which the juices, expressed from 

 the captured insect by the action of the maxillary plates (^fig. 109. ii) 

 were filtered, as it were, into the narrow oesophagus. In the Mygale, 

 however, there is certainly but one aperture : this is defended above 

 by a horny plate, or rudimental labrum ; below by the labium, which 

 is soldered to the jDlastron in the Mygale, but jointed and movable 

 in most of the smaller spiders. 



The pharyngeal fissure (yf^. 109. V) ascends between an anterior con- 

 vex plate or palate and a posterior concave plate, both which are shed 

 and renewed at each moult. The slender oesophagus passes back- 

 wards at a right angle to the pharynx, j^erforates the nervous ring, 

 and expands into the stomach {^Jig. 109. d). In the house-spider 

 ( Tegenaria domestica), the gastric cavity is produced into four sacs, 

 which are susceptible of great distension when a large prey is cap- 

 tured. In another species of spider, the oesophagus {Jig. 110. a), 

 having passed under the brain (c), suddenly expands into a stomach, 

 almost as broad as the sternum, which sends off a long clavate caecal 



s 



