62 



ORGANIZATION OF ARACHNIDANS. 



8. Most arach'nidans have a complete circulation. In these 

 animals the heart is placed in the abdomen, and in several species 

 of aranei'dse (from the Latin, ara'nea, a spider) its pulsations can 

 he distinguished through the teguments. It is a large longitudinal 

 vessel, which gives rise to the arteries and receives the veins 

 through which the blood returns from the respiratory organs to 

 be again distributed to different parts of the body. 



9. In this class of animals the 

 organs of respiration differ exceed- 

 ingly ; in some they consist of pul- 

 monary sacs, and in others, of 

 tra'chese. 



10. The pulmonary sacs (br,Jig. 

 56) are small cavities, the parietes 

 of which are formed by the union 

 of a great number of extremely 

 thin, white, minute triangular plates. 

 The number of these respiratory 

 pouches is generally two ; but some- 

 times there are four or even eight. 

 The apertures through which each 

 one communicates externally, called 

 stigmata or spiracles (s), are in form 

 of minute transverse slits, situate 

 at the inferior part of the abdomen. 



11. The tra'chece are tubes that 

 issue from or rather are continuous 

 with apertures similar to those jusl 

 mentioned, and are ramified through 

 the substance of all the organs, so 

 as to convey air to all parts of the 



body. This arrangement is represented in Jig. 13 (page 25), 

 which shows the arrangement in an insect. 



Explanation of Fig. 56. Anatomy of Arach'nidans. A mygale seen 

 from below. 7 1 , the ce'phalo-thorax ; A, the abdomen ; m, the mandibles ; 

 pa, the palpi of the jaws ; p \,p 2, p 3, p 4, bases of the legs ; gc y the 

 cephalic ganglion or brain, behind which we see the nervous collar which 

 surrounds the oesophagus ; gt, the nervous mass formed by the union of 

 the thoracic ganglia ; n, nerves of the legs ; ga, abdominal ganglion ; , 

 utigmata or spiracles ; 6r, one of the pulmonary sacs opened to show the 

 membranous laminae which line it internally ; o, the ovary ; on, the anus ; 

 f, the spinnerets. 



8. What kind of circulation have arach'nidans ? 



9. Is the character of the respiration the same in all arucli'iiidaus? 



10. What are pulmonary sacs ? What are stigmata ? 



11. What are tra'chem? (pronounced, tra'-ke-ay.) 



an f 



Fig. 56. ARACH'NIPAN. 



