124 CLASSIFICATION OP THE ANIMAL KINGDOM. 



more perfect organization, their sexual apertures are also situated beneath 

 the thorax or at the base of the abdomen, and their respiration is generally 

 carried on by means of lungs instead of tracheae. This class is divided 

 into the great groups — the Arachnida tracJiearea, which is an intermediate 

 type between insects and true Araehnidans, the Pedipalpi, and the Araneidoe, 

 the two last being also classed together under the name Arachnida pul- 

 monaria. The first division comprises the mites, (Acaridce,) and the 

 Pseudo-scorpionidce, Their bodies are divided into cephalo-thorax and 

 abdomen, they have eyes never exceeding four in number, and resembling 

 the eyelets of insects; their mouths are adapted to suction, and they have 

 strong piercing jaws; they breathe by means of tracheae, as in insects, 

 though their spiracles are different. Little is known accurately of their inter- 

 nal configuration, owing to their minute size. The Pulmonary Araehnidans, 

 comprising the two last divisions, are carnivorous in their habits. 



In the Pedipalpi, or Scorpions, the mandibles of the insect world are 

 represented by a pair of small forceps, and the maxillary palpi are very 

 greatly enlarged, and resemble the claws of Crustaceans. In the Spiders 

 the mandibles are terminated with a sharp moveable perforated fang, while 

 the maxillary palpi in the females terminate with a simple hook, and in 

 the males are provided with forceps. In both Scorpions and Spiders the 

 alimentary canal is very narrow, and is surrounded with a quantity of 

 fat, as in the larvas of insects. Their respiratory system is very peculiar, 

 being a combination of gills adapted to water, and lungs of air-breathing 

 animals. It consists of a series of pulmo-branchise, each of which opens 

 externally by a sort of spiracle; they possess a rudimentary vascular appa- 

 ratus for the circulation of the blood, which seems not to be confined in 

 veins, but wanders slowly in wide sinuses or cavities throughout the body, 

 and is thus brought back to the arteries, to be again propelled through 

 the system. The nervous system of Scorpions and Spiders differs, however, 

 somewhat in character. In the former, it is still ganglionic, though more 

 concentrated and enlarged than that of insects, each ganglion moreover is 

 united by three intermediate nerves; but in the Araneidte, or Spiders, we 

 find the whole chain of ganglia inserted into one* brain, from which nerves 

 radiate to all parts of the body. The eyes of Araehnidans are far supe- 

 rior to those of insects, and resemble greatly in their construction those 

 of vertebrate animals. Their sexual organs, both male and female, are 

 very simple in their character. The Araneidce, or Spiders, like the larvae 

 of insects, possess the power of spinning; but instead of a single prominence, 

 the former possess four spinnarets, each of which is perforated by innumer- 

 able orifices, so that, instead of a simple line, each thread is composed 

 of numbers of small cords woven together. 



The last class of the great Homogangliate division, the Crustacea, are 



