368 THE ARACHNOIDAE. <§,«§, 296, 297. 



CHAPTER I. 



EXTERNAL ENVELOPE AND CUTANEOUS SKELETON. 



§ 296. 



The external envelope of the Araehnoidae is usually soft, or coriaceous, 

 rarely horny ;<^> but in no instance does it possess a proper contractility. 

 In place of this, however, it is extensible in the highest degree with many 

 species. This extensibility is seen especially with those species which are 

 accustomed to long fasts, having only an occasional opportunity to fill their 

 digestive canal with food consisting of the animal juices.^-' 



The envelope is composed here, as with all the Arthropoda, chiefly of 

 chitine.*''* To this last are undoubtedly due its solidity and indestructibility, 

 which may be observed with the small and delicate Acarina and Tardi- 

 grada, not only when it is in a fresh state, but even after it has been cast 

 oflf by a kind of moulting. <'*> 



§297. 



With most Araehnoidae, the cutaneous envelope may be separated into 

 two tunics ; an external and an internal. The first is the more solid and 

 thick, and, in the cephalothorax and the extremities, has often a cellular 

 structure. Upon the abdomen of the Araneae and Acarina, it presents 

 peculiar, waving markings which, as concentric rings, surround the base of 

 the hairs ;*^' but it is difficult to determine if they are due to delicate 

 plicae, or the eft'ect of the intimate structure of the skin. With Ixodes, 

 only, these prominent lines appear, unmistakably, as folds of the epidermis, 

 for they completely disappear when these animals are gorged with food. 



The epidermis is often provided with papillae, clavate excrescences, 

 spines, bristles, simple or plumose hairs, and even, sometimes, with scales.'-* 

 These various cutaneous formations, which are usually hollow, either 

 occupy only certain points, or are extended over the whole surface of the 

 body, giving it a velvety or a furry aspect. 



The internal tunic of the skin consists of a thin, always colorless mem- 

 brane, finely granular or fibrillated, which is perforated at those points 

 where there are hair-like or other formations of the epidermis.'"' Directly 

 beneath this membrane, which, undoubtedly, reproduces the epidermis after 



1 For example, with the Scorpioniclae and Phry- saigne ( loc. cit.), and of that of Ascaris, Meck- 

 nidae. The cutaneous envelope is hardest and elia, Sabella, Hermione and Nephtys, made by 

 most fragile with the Oribatea, where it breaks like Loewig and KiiUiker (Ann. d. Sc. Nat. V. 1846, 

 glass from the lightest pressure. p. 198). 



2 For example, with Ixodes, and Argas, as also 1 For example, with Epeira, Segestria, Thom- 

 with the parasitic larvae of certain liydrachuea isu.i, Arsyroneta, Sa/ticim, Sarcoptcs, &c. 



and Trombidina, known under the names of Achly- 2 Plumose liairs are very often found with the 



sia and Leptus. Araneae ; and I have found lanceolate scales with 



3 Lassaigne, Compt. rend. XVI. 1843, No. 19, Sa/ticus, and clavate excrescences with the Trem- 

 or Froriep^s neue Not. XXVII. p. 8, wadSchmidt, bidina ; see Hermann, loc. cit. PI. III. fig. — Y. 

 Zur vergleich. Physiol, p. 47. 3 I am unable to say whether the internal mera- 



i This solidity of the skin with the Tardigrada, brane is prolonged at these points into the hollow 



is one evidence that these animals are more prop- excrescences of the skin, or whether the appear- 



erly classed with the Araehnoidae, instead of with ances alluded to are not produced artificially when 



the worms whose skin contains no chitine and is, the outer is separated from the inner layer of the 



therefore, (luickly dissolved in caustic potass. See skin, 

 the analyses of the skin of the earth-worm by Las- 



