178 ORGANS OF DIGESTION. 



testinal canal of hymenopterous insects we find in some genera 

 a venomous apparatus, consisting of a sting, a poison-bag, and 

 secreting glandular organs. In the bee the sting is situated on 

 the last segment of the abdomen, above the opening of the 

 rectum ; its base is surrounded by a small bag, embraced at 

 its superior part by numerous muscles ; two vessels, or caeca, 

 enter this reservoir with their poisonous secretion ; the sting 

 is composed of two portions, the corresponding surfaces of 

 which are grooved in a semilunar manner, so that, when ap- 

 proximated, a channel is formed ; into this the duct of the poi- 

 son-gland opens ; each half being armed with small sharp re- 

 curved teeth, for retaining it in the wound. The sting has a 

 sheath for its reception, and a particular set of muscles, under 

 the control of the will, for effecting its movements. 



Insects possess salivary vessels opening into different situa- 

 tions ; some pour their secretion into the mouth, others into the 

 commencement of the stomach (fig. 179). When we survey the 

 varied forms which the biliary organs assume in theinvertebrated 

 animals, we may remark that among the articulata, respiring 

 atmospheric air, these organs present an arrangement and 

 structure very different from that observed in the aquatic ar- 

 ticulata and mollusca ; we are thus led to study more particu- 

 larly the relations existing between the function of the liver 

 as a secreting organ, and the respiratory apparatus as an ex- 

 halant system; the latter rejecting from the economy car- 

 bonaceous matter in a gaseous form, whilst the liver is con- 

 stantly eliminating from the system secretions abounding in 

 carbon and hydrogen, with other greasy and resinous materials. 



[ 328. The vertebrate animals resemble man in the general 

 arrangement and division of the digestive organs (fig. 180) ; 

 their principal differences depending upon the nature of the 

 food ; the purely carnivorous species having a shorter and 

 simpler apparatus than those which are frugivorous : among 

 the latter the stomach is often a compound organ. In the ro- 

 dents, as the rat, there are two compartments, and in ruminants 

 four distinct cavities, whilst in the carnivora it forms a simple 

 bag, as in man. The intestinal canal bears a constant relation, 

 in its length and development, to the kind of food to be di- 

 gested. In general, the length of the intestine is greatest in 

 the ruminants, varying from fifteen to twenty times the length 



