ARACHNIDA. 



203 



Fig. 81, 



a 



claw, as in the spiders, at least the females, 

 for in the males this palp is frequently the seat 

 of a singular apparatus (e\ hereafter to be 

 described. Thirdly, of a sternal labium (f), 

 which, as its name implies, is inserted into the 

 sternum, and does not give origin to any arti- 

 culated appendage or palp. With respect to 

 the composition of the mouth in the parasitic 

 species, such as most of the mites, and we 

 may take as an example an argas, although 

 it is concealed under the form of a beak, 



sometimes with a sharp 

 point, yet it is essentially 

 the same. The principal 

 difference consists in the 

 dart - shaped mandibles 

 (a a), being joined toge- 

 ther so as to form a kind 

 of lancet, the sides of 

 which are sometimes 

 denticulated, so as to 

 cause them to adhere 

 firmly to the flesh which 

 they have penetrated. 

 The maxilla with their 



palp (6) and the inferior 

 Head of amitt( Argas.) labium ( c ) are here more 



or less intimately blended together, so as to 

 form a case or sheath. In some instances the 

 maxillary palp remains free, as in the argas. 



Savigny admits that in the interior of the 

 mouth of arachnidans there exist three pharyn- 

 geal orifices, and not a single one as in crus- 

 taceans and insects. These three orifices, 

 which are of almost imperceptible minuteness, 

 are situated at some distance from one another, 

 and disposed in a triangular form. He has 

 observed this structure in spiders, scorpions, 

 and phalangians : but he represents only two 

 orifices in a genus allied to galeodes. Latreille 

 denies the fact, and Treviranus, in his anato- 

 mical description of arachnidans, mentions 

 only one pharyngeal orifice. 



However this may be, Savigny confines the 

 assumption of food in spiders to a true suction : 

 " The mandibles," says he, " do not serve for 

 bruising the food, but for seizing it, and for 

 piercing and retaining it in firm contact with 

 the maxillae ; these subject it to alternate pres- 

 sure, and express the juices which afterwards 

 pass into the pharynx."* This is a matter 

 of daily observation when a spider seizes an 

 insect. 



The intestinal canal of the arachnidans is 

 always short, and is never disposed in convo- 

 lutions as in certain herbivorous insects. This 

 disposition is in accordance with their preda- 

 ceous habits, and confirms the general rule, 

 (but which to our knowledge is not without 

 many exceptions,) that the intestinal canal is 

 longer in herbivorous than carnivorous animals. 



In the spiders, (araneae,) and we may take 

 the common species (tegenaria domestica) as 



Fig. 82. 



c t 



jr.::. 



^ Description of Egypt, Arachnidans, pi. 8, 

 ' E y y- Savigny at first admitted but two 

 pharyngeal openings, (Memoir sur les Animaux 

 sans Vertebres, p. 57) ; but subsequently admitted 

 three. 



Tegenaria domestica. 



an example, the alimentary canal (Jig. 82) com- 

 municates with the mouth between the maxillae 

 (a a) by an oesophagus, rather short and of a de- 

 licate texture (6). This terminates in four sacs 

 (c), which M. Treviranus calls " stomach," but 

 which, in our opinion, merit rather the name of 

 gizzards ; the digestive tube then continues, 

 as a straight narrow canal (c?) of moderate 

 length, which dilates (e) and adheres, by its 

 parietes, to a kind of epiploon filled with adi- 

 pose granules (f). Posteriorly the dilated part 

 becomes stronger in texture, insensibly con- 

 tracts^), then undergoes a second dilatation (A) 

 before it opens into the rectum ('). It is near 

 the latter part, in a kind of pouch, that the 

 slender vessels open which M. Treviranus calls 

 biliary vessels, and which he is, with reason, 

 surprised to see terminating in so extraordinary 

 a position. These vessels, in fact, which cha- 

 racterize so well by their presence the chilific 

 stomach of insects, and are situated in these 

 animals more or less anteriorly, always pre- 

 ceding the small intestines which have a greater 

 or less length, terminate in the spiders in the 

 rectum itself, and close to the anus. 



