§ 298. THE ARACHNOIDAE. 369 



moulting, is a layer of colored vesicles and granules, which can be seen 

 through the skin, giving it the often very beautiful colors which are 

 observed in many species. 



The various divisions of the cutaneous skeleton, of which the number is 

 quite limited, have been so thoroughly studied in zoology, that they may 

 well be passed over here without notice. 



The Cephalothorax sends off from its inner surface, especially with the 

 Opilionina, and Araneae, various processes, which serve, as with the Crus- 

 tacea, as points for the insertion of muscles, and as septa between certain 

 organs. With the Araneae, they form, at the bottom of the cephalothorax, 

 a solid horizontal plate, — a kind of internal skeleton, which, before and 

 behind, is attached to the sternum by two tendinous ligaments. This plate 

 is deeply indentated on its anterior border, and furnishes points of inser- 

 tion for the muscles of the extremities, as well as for several other 

 parts. ^*' 



CHAPTER II. 



MUSCULAR SYSTEM AND OKGANS OF LOCOMOTION. 



§ 298. 



The voluntary muscles of the Arachnoidae are of a dirty-yellow color, 

 and, like those of the Crustacea, are distinctly striated transversely.*^' Their 

 general disposition agrees, also, with those of Crustacea.*"' 



The principal muscular masses are found in the cephalothorax, for here 

 arise, not only the muscles of the parts of the mouth, but also those of the 

 first article of the tactile organs and legs. With those species having an 

 unarticulated abdomen, the muscles of this part of the body differ from 

 those of Crustacea. For, directly beneath the skin, is a thin layer, com- 

 posed of numerous short, riband-like fibres, interlaced in various directions, 

 and frequently anastomosing with each other. *'^' Moreover, with many spe- 

 cies, there are, on both the dorsal and the ventral surface of the abdomen, 

 depressions of the skin, from which pass off small muscular bands, which 

 penetrate into the interior of the abdomen, and pass among the viscera. 



With the Araneae there is, generally, on each side of the ventral median 



4 This plate, already recognized by Lyonet (loc. Meckel, Syst. d. vergleich. Anat. III. p. 47 ; and 



cit. p. 405, PI. XXI. fig. 26), and by Treviranus for the muscles of Phalangium and Mygale, see 



(Bau d. Arach. Taf. II. fig. 23), has been described Talk, and fVasmann, loc. cit. The very compli- 



more exactly by Wasmann (loc. cit. p. 2, fig. cated muscular system of the Tardigrada is quite 



2-4). A similar, but rudimentary plate, exists, apparent from the transparency of these animals ; 



perhaps, with Pkalangium, and, as it Ues under see Doyire, loc. cit. p. 335, PI. XVII.-XIX. 

 the ventral cord, the muscles have the appearance 3 This cutaneous layer, already observed by 



of rising from this last ; see Tulk, loc. cit. p. 325, Treiiranus (Verm. Schrift. I. p. 9, Taf. I. fig. 3, 



or in Froriep^s neue Notiz. XXX. p. 136. a. n.), and by Brandt (Mtdiz. Zool. I. p. 88, Taf. 



1 The Tardigrada form an exception m this re- XV. fig. 8, a. a., or Ann. d. Sc. Nat. XIII. p. 180, 



spect, their muscles being smooth ; see Doyere, PI. IV. fig. 1, a. a.), with Epeira, has been con- 



loc. cit. p. 336. firmed by Tulk (loc. cit. p. 154) with Phalan- 



- For the disposition and arrangement of the gium, and described in more detjiil by H^as7nann 



muscular system of Scorpionidae and Araneae, see (loc. cit. p. 8, fig. 1, 8) with Mygale. 



