Characteristics of Arachnida. cxvii 



have four pairs of limbs, which correspond to the maxillary^ and 

 to the labial palps, and to the two anterior pairs of legs in Insects, 

 The fusion of head and thorax approximates the Arachnida to the 

 Crustacea^ and puts them into a position of contrast to the other 

 Arthropoda ; the Myriopoda furnishing" here^ as frequently else- 

 wherCj an example of a transitional arrangement, by having the 

 two anterior pairs of thoracic appendages subordinated functionally 

 to the oral appendages, though the segments carrying them are 

 not in them actually fused with the head. As the history of the 

 development of the nervous system in the Scorpion appears to show 

 that the first post-oral, which is the cheliferous appendage, corre- 

 sponds to the mandible, and not, as is ordinarily stated, to the first 

 maxilla of other Arthropoda; we may add that the Arachnida 

 resemble the Crustacea in a second point of external anatomy, 

 that of possessing a palp on the mandible, a structure never seen in 

 the air-breathing Arthropoda except as a rudiment in some Chilo- 

 j)odous Myriopoda. The lesser development of the respiratory 

 apparatus is a point of internal anatomy which distinguishes the 

 Arachnida from the other Arthropoda ; and the large development 

 of the hepatic organ, which may be considered to compensate for 

 this comparative deficiency, is another point of resemblance to 

 Crustacea. As peculiarities which may possibly be correlated with 

 these, and which certainly point in the same direction, we may 

 mention the power of repeatedly moulting ; of reproducing at those 

 periods limbs which have been detached or mutilated ; and of 

 producing offspring repeatedly in the adult state, which the Arach- 

 nida possess in common with Crustacea. The higher Arachnida 

 resemble the Ch'ilognatlm and the Crustacea, in the bilateral ter- 

 mination of their generative ducts on anteriorly-placed segments of 

 their body ; and they resemble the Chilognatha in the non-anasto- 

 mosis of their tracheae. The digestive tract of the Arachnida differs 

 from that of all other Arthropoda in having in many cases lateral 

 coeca appended to it, and prolonged into the interior of the limbs 

 and mandibular palps. Their respiratory system consists either of 

 tracheae alone ; or of the so-called ' lungs,^ which are sacciform 

 modifications of tracheae, alone ; or of both combined ; and in some 

 cases, such as those of certain parasitic families and orders, and of 

 the Tardigrada, a specialized air-breathing apparatus is wholly want- 

 ing. The tracheae when present have very ordinarily a fasciculate 



