$$ 296, 297. 
368 THE ARACHNOIDAE. 
CHAPTER I. 
EXTERNAL ENVELOPE AND CUTANEOUS SKELETON. 
§ 296. 
The external envelope of the Arachnoidae is usually soft, or coriaceous, 
tarely 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 
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 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 effect of the intimate structure of the skin. With Ivodes,. 
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 
oceupy 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- 
nidae. The cutaneous envelope is hardest and 
most fragile with the Oribatea, where it breaks like 
glass from the lightest pressure. 
2 For example, with Irodes, and Argas, as also 
with the parasitic larvae of certain Hydrachnea 
and Trombidina, known under the names of Achly- 
sia and Leptus. 
8 Lassaigne, Compt. rend. XVI. 1843, No. 19, 
or Froriep’s neue Not. XXVIL. p. 8, and Schmidt, 
Zur vergleich. Physiol. p. 47. 
4 This solidity of the skin with the Tardigrada, 
is one evidence that these animals are more prop- 
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 Ascaris, Meck= 
elia, Sabella, Hermione and Nephtys, made by- 
ert and Kélliker (Ann. d. Sc. Nat. V. 1846,. 
p- + 
1 For example, with Epeira, Segestria, Thom= 
tsus, Argyroneta, Salticus, Sarcoptes, &c. 
2 Plumose hairs are very often found with the- 
Araneae ; and I have found lanceolate scales with. 
Salticus, and clavate excrescences with the Trom- 
bidina ; see Hermann, loc. cit. P}. III. fig. O—Y. 
8 Iam unable to say whether the internal mem- 
brane is prolonged at these points into the hollow 
excrescences of the skin, or whether the appear- 
ances alluded to are not produced artificially when 
an outer is separated from the inner layer of the 
skin. 
