204 



ARACHNIDA. 



Now if the observations of M. Treviranus 

 are correct, and the four vessels which he de- 

 scribes are really analogous to the biliary tubes 

 of insects, we do not hesitate to consider all 

 the part which precedes and is intermediate to 

 them and the four sacs, as the stomach, or 

 chilific cavity. It would thus result, that the 

 tegenaria domestica would be deprived of an 

 intestine properly so called, and would pos- 

 sess no part destined to transmit along a greater 

 or less extent the residua of the digestive pro- 

 cess. And, indeed, such residua must neces- 

 sarily be very inconsiderable in an animal 

 which is sustained by juices, and these already 

 animalized. We are, indeed, led to this con- 

 clusion by the structure presented by the he- 

 mipterous insects which are nourished, like the 

 spiders, by suction, and which also have the 

 intestines, properly so called, so short that the 

 biliary vessels, which always accompany the 

 posterior extremity of the stomach, are found 

 close to the anus. We may form an idea of 

 this disposition by casting an eye over the 

 beautiful figures which our friend M. LeonDu- 

 four has just published in his " Anatomical and 

 Physiological Researches on the Hemiptera." 



Fig. 83. 



Scorpio. 



In the alimen- 

 tary canal of the 



a scorpions the 

 biliary vessels 

 d d are inserted 

 much higher up, 

 but this is not 

 the only pecu- 

 liarity which 

 the anatomy of 

 these animals 

 presents. Their 

 digestive tube 

 extends without 

 any marked di- 

 latation straight 

 from the mouth 

 () to the anus 

 which opens at 

 the extremity of 



c the tail. It pre- 

 sents in this 

 course a very 

 singular struc- 

 ture : five small 

 canals (/>) go off 

 at right angles 

 from either side, 

 above the place 

 of communica- 

 tion of the bili- 

 ary vessels, and 

 terminate by 

 ramifying in 

 the fatty masses 

 which make a 

 sort of epiploon 

 (c.) This tru- 

 ly remarkable 

 structure is not, 

 however, so an- 

 omalous as 

 might be sup- 

 posed,especially 



if we regard as coacums these kind of lateral ves- 

 sels. For the alimentary canal presents a still 

 more ramified condition in some crustaceans, 

 we would cite as an example the argulus studied 

 by Jurine;* and in another animal of the same 

 class which M. Milne Edwards and myselff 

 liave made known under the name of Nicothoe, 

 the intestinal canal sends out considerable 

 lateral prolongations. In the leech, and es- 

 pecially the Clepsina, there exist numerous 

 coecums. Lastly, certain minute arachnidans 

 (acaridse) are remarkable for analogous lateral 

 dilatations. It is to be observed that all these 

 beings are sustained by animal juices, and the 

 great part, for the better gorging of the same, are 

 fixed either momentarily or during their whole 

 life upon the body of their victim. 



We now come to speak of the epiploon and 

 the fatty globules which it contains. The fat, 

 or the substance which appears as such, is ex- 

 tremely abundant in the bodies of insects and 

 arachnidans. In the latter it assumes the form 

 of granular masses or globules of various co- 

 lours, and sometimes these are united together 

 by a thin membrane. In the araneae the fat is 

 especially abundant in the abdomen, of which, 

 indeed, it determines the form. The use of 

 this fatty apparatus cannot be mistaken, and it 

 has been placed beyond doubt by experiment, 

 that it supplies the place of nourishment to the 

 animal, either when the latter passes the winter 

 in a state of torpidity, like the hibernating ani- 

 mals, or when in particular seasons circum- 

 stances are not favourable for catching prey. 



Respiratory system. The division which has 

 been established in the class Arachnida of Pul- 

 monaries and Trachearies indicates that there 

 are in these animals two very different modes 

 of respiration. In both cases the atmosphere 

 penetrates to the interior of the body by orifices 

 situated on different points of the body, and 

 called stigmata. The stigmata of the Pulmo- 

 nary Arachnidans, and especially those of scor- 

 pions, are very conspicuous ; they occupy the 

 inferior part of the abdomen, and are four in 

 number on either side, (1, 2, 3, 4, fg. 84.) 

 They are in the form of narrow fissures, sur- 

 rounded as in insects with a circle of more 

 solid substance than the rest of the integument, 

 and to which we have given the name of pe- 

 ritrema. 



In the spiders (araneae) not only do they 

 differ in form but in number and position. 

 Treviranus counts four pairs in the thorax 

 above the insertion of the legs, four pairs on 

 the upper part of the abdomen, and one pair 

 on the lower surface ; the latter is the most 

 constant and important, (jig. 100, d.) 



The stigmata of the Tracheary Arachniduns 

 are less easy to be distinguished, more espe- 

 cially on account of the small size of the 

 species constituting a part of that group. 

 We have here carefully figured them in an 

 Acaroid species ( Ixodes Erinacei), where they 

 are situated below the sides and on the lower 

 part of the abdomen, (Jig- 85, a,) in shape like 

 a spherical tubercle, (Jig. 86, a,) perforated by 



* A nnales du Museum, torn. viii. p. 431. 1806. 

 t Annales dc - Sciences Naiurtlles, first series, 

 torn. ix. pi. 49. 



