250 THE MICROSCOPE AND ITS REVELATIONS. 



not identical with" it, by inoculating it with the Human parasite. The 

 Acarida are best preserved, as Microscopic objects, by mounting in one 

 or other of the 'media' described in 206. 



646. The number of objects of general interest furnished to the 

 Microscopist by the Spider tribe, is by no means considerable. Their 

 Eyes exhibit a condition intermediate between that of Insects and Crus- 

 taceans, and that of Vertebrata; for they are simple, like the ' stemmata' 

 of the former ( 626), usually number from six to eight, are sometimes 

 clustered- together in one mass, though sometimes disposed separately; 

 while they present a decided approach in internal structure to the type 

 characteristic of the visual organs of the latter. The structure of the 

 Mouth is always mandibulate, and is less complicated than that of the 

 'mandibulate' insects. The Respiratory apparatus, which, where devel- 

 oped at all among the Acarida, is tracheary like that of Insects, is here 

 constructed upon a very different plan; for the f stigmata ' which are usu- 

 ally four in number on each side, open into a like number of respiratory 

 sacculi, each of which contains a series of leaf-like folds of its lining 

 membrane, upon which the blood is distributed so as to afford a large 

 surface to the air. In the structure of the limbs, the principal point 



Fig. 438. 



Foot, with comb-like claws, of the common Spider (Epeira). 



worthy of notice is the peculiar appendage with which they usually ter- 

 minate; for the strong claws, with a pair of which the last joint of the 

 Foot is furnished, have their edges cut into comb-like teeth (Fig. 438), 

 which seem to be used by the animal as cleansing-instruments. 



647. One of the most curious parts of the organization of the Spiders, 

 is the ' spinning-apparatus ' by means of which they fabricate their elab- 

 orately constructed webs. This consists of the * spinnerets,' and of the 

 glandular organs in which the fluid that hardens into the thread is elab- 

 orated. The usual number of the spinnerets, which are situated at the 

 posterior extremity of the body, is six; they are little teat-like promi- 

 nences, beset with hairy appendages; and it is through a certain set of 

 these appendages, which are tubular and terminate in fine-drawn points, 

 that the glutinous secretion is forced-out in a multitude of streams of 

 extreme minuteness. These streams harden into fibrils immediately on 

 coming into contact with the air; and the fibrils proceeding from all the 

 apertures of each spinneret coalesce into a single thread. It is doubtful, 

 however, whether all the spinnerets are in action at once, or whether 

 those of different pairs may not have dissimilar functions; for whilst the 

 radiating threads of a spider's web are simple (Fig. 439, A) those which 



