ARACHNIDA. 



207 



a. , : 



rectly termed the 

 tail. 



In the Aran- 

 idce the ganglions 

 are fewer than in 

 the Scorpionidte : 

 d the first pair, or 

 that which consti- 

 d tutes the brain, 

 (fig. 9 1, a,) is quite 

 distinct from the 

 e thoracic ; these are 

 four in number (bb) 

 but have under- 

 gone a remarkable 

 degree of centrali- 

 zation, being inti- 

 mately connected 

 together so as in- 

 deed to form a 

 mass in which all 

 traces of junction 

 are lost, except at 

 3 the sides, which 

 have remained free 

 and in the form of 

 smallconoid bodies 

 directed outwardly 

 so as to resemble, 

 in the aggregate, 

 the figure of a star. 

 From the apex of 

 each of these small 

 cones the nerve is 

 given off to each 

 leg. In the abdo- 

 men there does not 

 exist any ganglion, 

 but only a double 

 longitudinal ner- 

 vous cord(c),which 

 swells out at its ter- 

 mination. From 

 this swelling (d) a 

 great number of 

 nerves (ee) pass off, 

 which are distri- 

 buted to all the 

 organs contained 

 in the abdominal 

 cavity. 



Organs of sense. 

 We have no- 

 thing particular to 

 observe with re- 

 spect to the smell or hearing of the arachnidans, 

 for we are ignorant of the existence of these 

 senses in the class, or at least of the parts of 

 the body in which they are seated. With re- 

 gard to taste, the choice which the arachnidans 

 make of their food sufficiently indicates that it 

 exists in variable degree ; the organ is situated 

 probably at the entrance of the pharynx. 

 With regard to touch, the delicacy of that sense 

 is in the ratio of the tenuity of the integument; 

 but the extremities of the legs, and more 

 especially of the maxillary palps seem to 

 be expressly destined to bring the individual 



a 



Fig. 91. 



into relation with sur- 

 rounding objects. The 

 sense of sight is the 

 only one respecting 

 which no doubt can 

 exist; particularly in 

 the species which are 

 the most perfect of the 

 class, such as the spi- 

 ders, scorpions, &c. 

 The eyes belong to 

 that kind which are 

 termed simple, in op- 

 position to those de- 

 nominated compound, 

 and which are found 

 exclusively in insects 

 and crustaceans. 



These simple eyes 

 (ocelli) in arachnidans 

 are two, four, six, or 

 eight in number ; they 

 are situated on the an- 

 terior part of the body 

 either superiorly or la- 

 terally. With respect 

 to size they differ not 

 only in different spe- 

 cies, but in the same 

 individual, as in the 



Platyscelum, (Jig. 92,) and especially in the 



A tins. 



Fig 92. 



In the Scorpions (fig. 93) there are two 

 eyes (a a) situated on the dorsal aspect of the 



Fig. 93. 



cephalo-thorax, and closely approximated to 

 the mesial line : these are of much larger size 

 than the minute simple eyes (b 6), which are 

 placed on the sides and near the outer margins 

 of the same segment. The two mesial eyes, 

 on account of their size, have been selected by 

 Miiller for the subject of his researches, which 

 he published at Leipsic, and which have been 

 translated by extract in the 17th volume of the 

 first series of the " Annales des Sciences Natu- 

 relles." The following are the principal re- 



