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}. 
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 
hard 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 
humour 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 ¢ris. 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 Telyphonus, 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 here, 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 Lyoner?. It was also found by me in Phrynus, and in the 
1 Branpt Med. Zool. i. s. go, Newport |. 1. p. 261. 
* Recherches sur V Anat. &c. Ouvrage posthume. p. 100, Pl. 10, fig. 26, TREVIRANUS 
Ueb. d. inn. Bau der Arachn. s. 30, Tab. 1. fig. 23. TREVIRANUS calls this part 
incorrectly a cartilage (Knorpel). 
