EYES, FEET, AND SPINNERETS OF SPIDERS. 687 



mingled with it. These are to be separated by the addition of Oil, 

 which will probably soften the Sebaceous matter sufficiently to set 

 free the Animals, which may be then removed with a pointed 

 brush ; but if this mode should not be effectual, the Fatty matter 

 may be dissolved-away by digestion in a mixture of Alcohol and 

 Ether. The pustules in the skin of a Dog affected with the ' Mange' 

 have been found by Mr. Topping to contain a Demodex, which 

 seems only to differ from that of the Human Sebaceous follicles in 

 its somewhat smaller size ; and M. Grruby is said to have given to 

 a Dog a disease resembling the Mange, if not identical with it, by 

 inoculating it with the Human Parasite. — The JLcarida are best 

 preserved as Microscopic objects, by mounting in one or other of the 

 ' Media ' described in § 163. 



535. -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 In- 

 sects and Crustaceans, and that of Yertebrata ; for they are single, 

 like the 'Stemmata' of the former (§ 516), usually number from six 

 to eight, are sometimes clustered-together in one mass, but are some- 

 times 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 developed at all among 

 the Acarida, is Tracheary like that of Insects, is here constructed 

 upon a very different plan ; for the 'Stigmata,' which are usually 

 four in number on each side, open into a like number of Respira- 

 tory Saceuli, 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 worthy of notice is the peculiar appendage 

 with which they usually terminate ; 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. 353), which seem to be used 

 by the animal as cleansing-instruments. 



536. One of the most curious parts of the organization of the 

 Spiders, is the 'Spinning-Apparatus' by means of which they 

 fabricate their elaborately constructed webs. This consists of the 

 'Spinnerets,' and of the Glandular organs in which the fluid that 

 hardens into the thread is elaborated. The usual number of the 

 Spinnerets, which are situated at the posterior extremity of the 

 body, is six; they are little teat-like prominences, 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 imme- 

 diate^ on coming into contact with the air ; and the fibrils pro- 

 ceeding from all the apertures of each Spinneret coalesce into a 



