ARACHNIDS. 569 



nervus sympatheticus (p. 278), are only imperfectly recognised. 

 There has been found, in spiders and scorpions, a nerve that runs 

 towards the stomach, and arises by two thin roots from the posterior 

 margin of the cerebral ganglion 1 . 



The organs of sense in arachnids are limited, as far as they are 

 known, to those of sight and feeling. A distinct organ of hearing 

 has not yet been discovered. The seat of taste, probably also of 

 smell, is to be sought for perhaps in the commencement of the intes- 

 tinal canal. For feeling the palpi, in many at least, officiate ; in 

 others, where, as in Scorpio and Chelifer, they are covered with a 

 bard skin and are shaped like shears, they serve for seizing prey. 

 The long feet also of spiders and of Phalangia, and especially the 

 thread-like feet of the first pair in Phrynus, are peculiarly adapted 

 for organs of feeling. The eyes are constantly simple. Behind the 

 convex cornea is placed a very round lens, which is received by a 

 vitreous humour concave on the anterior surface. Behind this 

 tmmour lies the retina, surrounded by a coloured pigment of the 

 choroidea, which at the fore part, between the lens and the vitreous 

 humour, forms a ring like an iris. For the presence or absence and 

 the number of simple eyes, we refer to the systematic arrangement 

 of the class. Often, as in Scorpio, Phrynus and Tdyphonus, two 

 simple eyes are placed close together in the middle, and on each 

 side two groups of smaller simple eyes towards the margin of the 

 cephalothorax. If this disposition be compared with that of insects, 

 then the middle eyes may be likened to their simple eyes, and the 

 two lateral groups to their two compound eyes. 



The muscles of the arachnids are principally inserted in the 

 external integument. The cavity of the cephalothorax is especially 

 occupied by a mass of muscle, because the limbs, which are moved 

 by the muscles, are placed in a circle round about that part. In 

 spiders there is found liere, for the attachment of the muscles, in 

 addition, an internal oblong plate, tendinous, hollow above, and 

 with the anterior margin excised. This part was already observed 

 by LYONET 2 . It was also found by me in Phrynus, and in the 



1 BRANDT Med. Zool. 11. s. 90, NEWPORT 1. 1. p. 261. 



2 Recherches sur I'Anat. &c. Ouvraye posthume. p. 100, PI. 10, fig. 26, TUEVIRANUS 

 Ueb. d. inn. Ban der Arachn. s. 30, Tab. n. fig. 13. TREVIRANUS calls this part 

 incorrectly a cartilage (KnorpeT). 



