376 THE ARACHNOIDAE. ^ 306. 



brownish black, as with the other Arachnoidae ; but with the nocturnal 

 spiders, it is replaced by a membrane which has a splendid lustre.*"' With 

 the Phrynidae, there are also eight stemniata, of which two are situated on 

 the middle of the cephalothorax, and the remaining six form a triangle 

 composed of three on each of its sides. 



With the Scorpionidae, the eyes are the most numerous. There are two 

 large eyes on the middle of the cephalothorax, then a row of from two to 

 five smaller on each side of its anterior border. 



The number of optic nerves depends, usually, upon that of the eyes. 

 But the Scorpionidae form an exception in this respect ; for their brain 

 sends oiF, at the side of the two median optic nerves, two other nerves, 

 common, ;uid belonging to the two rows of marginal stemmata, but which 

 do not divide until they have reached these organs.'^"' On account of the 

 usually deep position of the brain, the optic nerve is generally of consid- 

 erable length ; but the Pycnogonidae alone differ in this respect from the 

 other Arachnoidae, for, with Phoxichilus, the four eyes are situated directly 

 on the brain, and, with Ammothea, this last sends off, as a common optic 

 nerve to the four eyes, a large, short prolongation.*"' 



CHAPTER V. 



DIGESTIVE APPARATUS. 



§ 306. 



The entrance of the digestive canal is surrounded by very variable 

 organs, but, with all, the Mandibles are always wanting. The organs usually 

 called such are only antennae metamorphosed into prehensile and masti- 

 catory parts. This is shown not only from the cerebral origin of their 

 nerves, but by the fact that they, or more properly the Cheliceres, never 

 act, like the mandibles of the other Arthropoda, in a horizontal direction. 

 Most of the Arachnoidae live on liquid food, and, therefore, the basilar 

 article of the maxillae is more or less abortive, and is rarely used in mas- 

 tication, while the succeeding articles are changed into a usually very 

 large tactile or prehensile palpus. 



In general, the organization of the parts of the mouth with the Arach- 

 noidae may be divided into the following five types : 



1. With the Tardigrada, there are real organs for suction. These con- 

 sist of a kind of sucker, situated on the end of a fleshy proboscis which can 

 be retracted into the head. On each side of this proboscis there are two 

 stylets (teeth) which, by means of a special muscular apparatus, can be 

 protruded into the former.'^' 



2. With most of the Acarina, the two cheliceres are sometimes forficu- 

 late or unciform, sometimes cultrate or styliform, and by their use, these 



9 Dugis, Ann. d. Sc. Nat. VI. p. 175. U Quatrefages, loc. cit. p. 77, PI. I. fig. !»•. 



10 Treviranus, Zeitsch. f. Physiol. IV. p. 92, 2»-. 



Taf. VI. fig. 3 ; and Mailer, Zuv vergleioh. Phys- 1 See Doyire, loc. cit. p. 319, PI. XIII.-XV. 

 iol. &c. p. 321, Taf. VII. fig. 10, or Ann. d. Sc. 

 Nat. XVII. p. 2::;8, PI. XVII. fig. 3. 



