120 ZOOLOGY. 



Tlie nervous system is much concentrated, being principally composed of a 

 large ganglion in the centre of the cephalothorax, before which are two others 

 with branches to the eyes and mouth. The principal ganglion has branches 

 to the abdomen and feet. In the scorpions, instead of the great central 

 ganglion, there are two rows of small ones, each united by a nervous cord. 



The class is divisible into the three sections, Aporobranchia^ for Pycno- 

 gonuon^ &c., in which there are no special breathing organs; Trachearia^ 

 in which breathing is effected by means of tracheae ; and Pulmonaria^ in 

 which the gills i-esemble the leaves of a book, and are adapted to breathing 

 air only. The first section contains one, and each of the others two orders. 

 The orders are named from the characters furnished by the body. The 

 Podosoiaata^ Leach, have the feet and body much alike; the Monomeroso- 

 tnata, Leach, have only one segment apparent ; in the Adelarthrosomata, 

 Westwood, the segments are uncertain ; in the Polynurosomata, Leach, 

 they are numerous, as in the scorpion ; and in the Dimerosomata^ Leach, 

 including the common spiders, the body is divided into two portions. 



Order 1. Podosomata. These animals are placed among the Crustacea 

 by Milne Edwards, because they have not the organs of respiration of the 

 Arachnida, but respire by means of the skin, like some of the lower Crus- 

 tacea, a character which some of the Hj'drachnidjE have, although no one 

 would pretend to remove them to the Crustacea on this account. In form, 

 the animals of this order approach to the crustacean genus Cyamits {pi. 

 78, fig. 24), although they have but eight feet like the Arachnida. These 

 are long and slender, and composed of eight articulations, including the 

 claw. The head, or rather the rostrum, is lengthened, and either cylindrical 

 or conical, without appendages, and the mouth is terminal and tri-lobed. 

 The thoracic portion can be distinguished from the snout, which character 

 distinguishes them from the great mass of the Arachnida, and it is composed 

 of four segments, followed by a small abdomen. There are four eyes upon 

 an eminence, situated upon the upper surface of the first segment of the 

 thorax, and as the snout does not, in the opinion of Erichson and Kröyer, 

 comprise the whole head, this must be looked for in the first segment of the 

 thorax, which often bears a pair of cheliform organs, corresponding, in the 

 view of Latreille, to the cheliform mandibles of Scorpio. The female (and 

 in some cases the male also) has attached to the first segment of the thorax 

 a pair of appendages much like the feet (but much smaller in size, and 

 without a terminal claw), used to support the bunches of eggs, and which 

 may be considered maxillae. As Erichson regards the segment next to the 

 rostrum as the head, he names its feet a third pair of jaws, whilst Kröyer 

 considers them feet, on the ground that the segment to which they are 

 attached is made up of the posterior part of the head (including the eyes) 

 united without a division to the anterior part of the thorax. 



The alimentary canal is straight, and it is peculiar in having lateral 

 tubular branches penetrating far into the feet, which disj)enses with the 

 necessity of having a regular circulation. In the more typical Arachnida, 

 examples are found of a stomach with branches, but they do not enter the 

 feet. 



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