174 ORGANS OF DIGESTION. 



laginous skeleton, which renders it better adapted for bruising 

 the aliments ; the framework is composed of five semi-osseous 

 pieces, provided internally with five teeth, surrounding the 

 pylorus ; three are large and two are small, being a repetition 

 of the type of organization we have already described in some 

 mollusca ; the several plates of this skeleton are moved by 

 muscles, so as to render it a powerful organ for bruising and 

 fracturing the shells of the smaller mollusca, on which 

 the Crustacea prey ; the calcareous parts of the stomach, 

 like the external shell, are periodically cast off ; the intes- 

 tine forms a straight tube, extending from the pylorus to 

 the tail, and terminating at the under surface of the central 

 plate. 



[ 326. In the ARACHNIDA, as the common domestic spider 

 (Teyenaria domestica), the mouth is provided with a pair of 

 mandibles, armed with sharp claws, a venomous apparatus, 

 and maxillse or jaws ; the mandibles are used for seizing, 

 wounding, and retaining prey, whilst with the maxillse they 

 squeeze out and suck the contained juices of their victim. 

 The esophagus is short, of a delicate texture, and opens into 

 four crops, or stomachs ; the tube then continues a straight 

 and narrow canal, soon expanding into a muscular organ, sur- 

 rounded by numerous adipose granules ; this dilatation again 

 contracts, and, before terminating in the rectum, undergoes 

 another swelling ; into this enlargement the biliary vessels ter- 

 minate ; the apparatus for spinning is formed of four hollow 

 cylinders, the inferior parts of which are perforated like a 

 sieve, their superior apertures communicating with ducts, from 

 ramified vessels, destined for the secretion of the viscous fluid 

 forming the filaments of the web ; these tubes occupy a con- 

 siderable portion of the abdomen, surrounding the termination 

 of the intestine, and their sole function being the secretion of 

 this fluid. 



[ 327. In INSECTS (fig. 1/9) the digestive organs are ex- 

 ceedingly varied and complicated ; in some the mouth is pro- 

 vided with jaws for bruising (fig. 195), in others with an 

 apparatus for sucking (fig. 196) ; the intestinal canal presents 

 many enlargements, and, in some orders, is extremely con- 

 voluted, terminating at the posterior part of the body ; there 

 are distinct organs for the secretion of the bile and the saliva, 

 and in some a rudimentary pancreas exists. Insects pass 



