§*§. 300. 301. THE ARACHNOID AE. 871 



CHAPTEll III, 



NERVOUS SYSTEM. 



§ 300. 



The grades of development of the Nervous System with the Arachnoidae 

 are very different, being connected with the divisions of the cutaneous skel- 

 eton. For, when these last disappear, those of the nervous system belong- 

 ing to them, and often the ventral cord, are concentrated, as with the 

 brachyurous Decapoda, into a single ganglionic mass, occupying the ventral 

 portion of the cephalothorax ; while, if the body is multi-articulate, this sys- 

 tem resembles that of the macrurous Decapoda. In both cases, with only 

 a few exceptions, there is a cerebral ganglion situated above the oesophagus, 

 and connected with the ventral cord by two short commissures surrounding 

 this canal. From this ganglion pass off nerves to the eyes, and the maxil- 

 lary palpi or so-called mandibles ; while the first pair of maxillae, changed 

 into tactile organs, receive their nerves from the anterior extremity of the 

 ventral cord. 



The intimate structure of the nervous system, with the Arachnoidae, con- 

 sists of primitive fibres much finer, and ganglionic globules much smaller, 

 than those of Crustacea.'^* As to the direction and disposition of these 

 fibres, those of the Scorpionidae almost exactly resemble those of the 

 Myriapoda.'-^ 



§ 301. 



The nervous system is most simplg in its organization with the Acarina. 

 In those species where, as yet, it has been found, "^ it consists only of a 

 simple abdominal ganglion, from which pass off, from all sides, the peri- 

 pheric nerves ; and, upon the upper surface of which, is detached a simple 

 transverse band, under which the oesophagus passes. 



With the Tardigrada, this system is a little more developed, although 

 the brain is still wanting.*-* It consists of four ganglia, corresponding to 

 the four segments of the body, and connected together by double longitudi- 

 nal commissures. Between each of the ganglia, the commissures are con- 

 nected by a transverse filament. The nerves which proceed from the 

 ganglia belong to the muscles ; but the first ganglion sends, moreover, in 

 front, four larger trunks, which are the nerves of sense, and are distributed 

 to the eyes and palpi, ^'^^ 



1 Hannover, loc. cit. p. 71, PI. VI. fig. 83, 84. confirmed with tliis genus and with Limnochares, 



- See § 271, and Newport, Philos. Trans. 1843, by Dujardin (Ann. d. Sc. Nat? III. p. 19). Sub- 



loc. cit. seciueutly, Treviranus (Zeitsch. f. Physiol, loc. 



1 With many small Acarina, particulaily Sar- cit. p. 189, Taf. XVI. fig. 7. c.) has also con- 



cnptes and Demodex, no traces of a nervous sys- firmed, with Ixodes, this passage of the oesophagus 



tem have been found, notwithstanding the most througli the principal ganglionic mass. With 



Ciireful researches ; but this is not surprising, con- Trombidium, whose ganglion is somewhat reddish, 



«idering the minuteness of these animals. the cerebral commissure is quite distinct. 



■i Treviranus (Verm. Schrift. 1. p. 47, fig. 32) 3 See Doyire, loc. cit. p. a43, PI. XVII. (Milne- 



lias investigated the nervous system of Trom- stum), 

 itidium, and the results he obtanied liave been 



