S68 THE ARACHNOIDAE. §<§> 296, 297^ 



CHAPTER I. 



EXTERNAL ENVELOPE AND CUTANEOUS SKELETON. 



§ 296. 



The external envelope of the Arachnoidae 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 till 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 Acariifti and Tardi- 

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

 ofi by a kind of moulting.**^ 



§297. 



With most Arachnoidae, 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 efiect 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 Scorpionidae and Phry- saiane ( loc. cit.), and of that of Jscaris, Meek- 

 nidae. The cutaneous envelope is hardest and elia, Sabella, Hermione and Nephty.i, made by- 

 most frai.'ile with the üribatea, where it breaks like Loemig and KoUiker (Ann. d. Sc. Nat. V. 184(3, 

 glass from the lightest pressure. p. 198). 



^ For example, with Ixodes, and Arsras, as also 1 For example, with Epeira, Sege.siria, Thorn- 



with the parasitic larvae of certain llydrachnea isus, Arf^yroneta, Salticus, Sarcoptes, kc. 

 and TrMnil)i(lina, known under the names of .4cA/i/- 2 Plumose liairs are very often found with the 



tia anil Lt//tus. Araneae ; and I have found lanceolate scales with 



8 Ld-tsdii^ne, Compt. rend. XVI. 1843, No. 19, Sa/ticus, and clavate e.xcrescences with the Trom- 



or Froriep\<i neue Not. XXVII. p. 8, aadSc/imidt, bidina ; see Hermann, loc. cit. PI. III. fig. — Y. 

 Zur vergleich. Physiol, p. 47. 3 1 am unable to say whether the internal meni- 



* This solidity of the skin with the Tardigrada, braue is prolonged at these points into tlie hollow 



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



erly classed with the Arachnoidae, instead of with anccs alUuled to are not produced artificially when 



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



therefiire, quickly dissolved in caustic potass. See skin. 

 tUe aualyses of the likin of the earth-worm by Las- 



