36 



ARACHNIDA. 



by the fact that the abdomen consists of ten segments visible 

 externally. It is a striking fact that the Solifugae, which breathe by 

 means of dendriform tracheae, possess at least three pairs of stigmata; 

 the first opens on the fourth segment of the body, viz., the second 

 thoracic (i.e., the first free thoracic) segment; the second pair opens 

 on the second abdominal, and the third pair, which are closely 

 approximated, open on the third abdominal segment. A fourth 

 opening may be present as a median stigma on the fourth abdominal 

 somite.* 



We cannot agree with those who find actual relationship to the 

 Insecta implied in the very striking features we have mentioned, and 

 regard the Solifugae as a connecting link between the two stocks of air- 

 breathing Arthropoda. The value of a division of the anterior body 

 into head and thorax, in which the three anterior pairs of limbs 

 would have to be considered as the equivalents of the three pairs of 

 oral limbs in the Insecta, is diminished by the fact that one pair is 

 still wanting, i.e., there is in the Solifugae no homologue for the 

 antennae of the Insecta. The most difficult point to explain is the 

 position of the pair of stigmata on the cephalo-thorax ; we can only 

 assume that it was acquired later. The assumption gains in proba- 

 bility when we find that stigmata appear on the cephalo-thorax in 

 the Acarina also, on the legs in Opiliones (Hansen), and on the head 

 in Scolopendrella and Sminthurus (?). The presence of a spiral 

 filament in the tracheae of the Solifugae is no proof of their 

 relationship to the Insecta, since it occurs also in other Arachnida. 



In spite of the external division of the body into three parts, the 

 Solifugae agree so closely with the Arachnida in outer and inner 

 organisation, that we are not justified in separating them from that 

 class. The shape of the chelicerae, the possession of a coxal gland, 

 like that which is found in the Arachnida (MacLeod, No. 44), the 

 hepatic tubules derived from the enteron,f the position of the genital 

 aperture on the first abdominal segment, and other less striking 

 features favour the Arachnid character of the Solifugae. We there- 

 fore regard them as a branch of the Arachnid stock developed in 

 a special direction, a view which corresponds to that of Ray 

 Lankester (No. 45) and other writers on this subject. The slight 



* [Beunakd, op. cit.] 



t With regard to the liver, it should lie mentioned that a more recent 

 observer (Biiujla, No. 42) has found certain differences in structure between 

 this organ in the Solifugae and the Arachnida in general. He also, however, 

 describes the liver as a well-developed organ filling up the interstices between 

 the other organs, a description which applies to the liver of an Arachnid, but 

 not to that of an Insect. 



