ARACHNIDA PULMONARIA SPINNERS. 63 



12. Those arach'nidans that breathe by these tubes have no 

 circulatory apparatus, while those that breathe by lungs are 

 always provided with one. 



13. After leaving the egg, these animals do not, like insects, 

 indergo metamorphosis, although at this period they often have 

 but six legs, the fourth pair not being developed until after the 

 little creature has changed its skin ; like the crusta'ceans, the 

 arach'nidans frequently cast the skin or moult. 



14. The class of arach'nidans is divided into two orders, which 

 may be distinguished by the following characters : 



1st. The ARACH'NIUA PULMONA'RIA have eight simple eyes, 

 and pulmonary sacs for respiration. 



2d. The ARACH'NIDA TRACHEA'RIA have at most four simple 

 eyes, and trach'ess for respiration. 



ORDER OF ARACH'NIDA PULMONA'RIA. 



15. The division of pulmonary arachnidans includes all the 

 common araneidaB. The circulatory apparatus is well developed, 

 and they have from six to eight eyes, while the next order has 

 but four or even only two. The number of stigmata is two, four, 

 or eight. 



16. This group is divided into two families : the Aranei'dce or 

 spinners, and the Pedipalpi. 



17. The ARANEI'U^E or spinners have but one or two pairs of 

 pulmonary cavities, which may be distinguished by as many 

 whitish or yellowish spots near the lower part of the abdomen ; 

 their palpi are in form of little feet without pincers at their 

 extremity (fig. 56, p). 



18. One of the most curious phenomena in the history of these 

 animals is their mode of spinning silk, and with this delicate 

 material making webs which are as remarkable for their extent 

 as for the regularity with which they are woven. This silk is a 

 matter secreted by a peculiar apparatus situated in the abdomen 

 of the spider; it escapes externally by a certain number of spin- 

 nerets or small holes placed at the summits of several little nip- 

 ples near the anus (/, f-g. 56). The threads of silk at the 

 moment of escaping are glutinous, and to be employed by the 

 animal, require to be dried, but when the temperature is favour- 



12. Are trach'eoe in arachnidans accompanied by a circulatory apparatus ? 



13. Do arach'nidans experience metamorphosis ? 



14. How is the class of arach'nidans divided ? 



15. What are the characters of the pulmonary arach'nidans? 



16. How are the pulmonary arach'nidans divided? 



17. How are the aranei'dae distinguished ? 



18. What is spiders' web ? How is it formed ? To what purposes is it 

 applied ? 



