I 



368 



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 fill their 

 digestive canal with food consisting of the animal iuices/-' 



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 

 off 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 mai'kings 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 effect of the intimate structure of the skin. With Ixodes, 

 only, these prominent lines apjpear, 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 



I 



1 For example, with the Scorpionidae and Phry- 

 nidae. Tlie cutaneous envelope is hardest and 

 most fragile with the Oribatea, where it breaks like 

 glass from the lightest pressure. 



2 For example, with Ixodes^ and Argas, as also 

 with the parasitic larvae of certain Hydrachnea 

 and Trombidina, known under the names of Aclily- 

 sia and Leptus. 



3 Lassaigne, Compt. rend. XVI. 1843, No. 19, 

 or Froriep's neue Not. XXVII. p. 8, a,ndSchmidt, 

 Zur Tergleich. Physiol, p. 4"^. 



i This solidity of the skin with the Tardigrada, 

 is one evidence that these animals are more proj)- 

 erly classed with the Arachnoidae, instead of with 

 the worms whose skin contains no chitine and is, 

 therefore, quickly dissolved in caustic potass. See 

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



saigne ( loc. cit.), and of that of Ancaris, Meck- 

 elia, Sabella, Hermione and Nephtys, made by 

 Loewig and Kolli/cer (Ami. d. Sc. Nat. V. 1846, 

 p. 198). 



1 For example, with Epeira, Segestria, Thom- 

 isus, Argyroneta, Salticus, Sarcoptes, &c. 



2 Plumose hairs are very often found with the 

 Araneae ; and I have f jund lanceolate scales with 

 Sa/ticus, and clavate excrescences with the Trom- 

 bidina ; see Hermann, loc. cit. PI. III. fig. — Y. 



3 I am unable to say whether the internal mem- 

 brane is i)rolonged at these points into the hollow 

 excrescences of the skin, or whether the appear- 

 ances alluded to are not produced artificially when 

 the outer is separated from the inner layer of the 

 skin. 



