Tertiary Spiders and Opilionids. 217 



carapace represents apparently not a fusion of the head with the 

 thorax alone, but includes the first and second abdominal tergites. 

 The segmentation of the scorpion shows the appearance of a spuri- 

 ous segment, but is in other respects similar to that of spiders. 

 The carapace in all Arachnida represents the fusion of the tergites 

 of the head and thorax, unless one or more of the thoracic somites 

 are free, but it never includes any of the abdominal tergites. The 

 arterial system is entirely independent of the nervous system and 

 composed of supraneural, subneural and interneural arteries. 

 Xiphosura cannot be regarded as ancestors of Arachnida for the 

 same reasons. Very likely Xiphosura have Trilobites for ances- 

 tors as is also suggested by Raymond in his extensive monograph. 

 The relationship of the Gigantostraca (Eurypterida) cannot be 

 determined at present without further research. Provisionally at 

 any rate they may be safely left with the Arachnida. 



The new definition of the Class Arachnida may therefore be 

 given in the following words. Arachnida are Arthropoda with 

 heteromeric segmentation. Their body is composed of 21 somites 

 four of which are prae-oral in the adult, the fourth somite bearing 

 the chelicera. The first postoral somite belongs to the head and 

 bears the pedipalpi. The thorax consists of four somites each 

 provided with a pair of appendages, with the exception of some 

 Acari in which only the first and second thoracic somites bear 

 appendages. Head and thorax are usually fused into a cephalo- 

 thorax, but in some Orders one, two or even three of the thoracic 

 somites may remain free. The al^domen is composed of 12 

 somites, the first of which is rudimentary in the adult, being rep- 

 resented only by its neuromere, except in the Pedipalpi in which 

 it persists throughout life. Further fusion of somites involves 

 the posterior end of the abdomen, beginning with the last somite 

 and proceeding forward. Secondary spurious segmentation in the 

 abdomen, as in the case of the i8th somite of the scorpion, may 

 result in the formation of an additional segment which does not 

 possess the value of a somite. The anus is always on the last 

 somite. The genital opening is on the eleventh (second abdomi- 

 nal) somite. Nephridia modified as coxal glands situated in the 

 thorax. Respiration by means of lung-books or tracheae, in some 

 forms organs of respiration entirely absent. Circulatory system 

 with a heart in which the cardio-aortic valve always marks the 



