GENERAL CONSIDERATIONS REGARDING THE ARACHNIDA. 115 



There are various other points of organisation in which the Arach- 

 nida are removed from the Insecta, but approach the Xiphosura, 

 and perhaps even the Crustacea. 



In dealing with the eyes, we tried to show that they cannot be 

 classed together with those of the Insecta and the Myriopoda, but 

 have had a different course of development (p. 68). They may, 

 however, well be homologised with the median and lateral eyes of 

 Lunulas. In the origin of the Arachnid eyes, inversion plays an 

 important part. Inversion has recently, also, been introduced by 

 Claus as an explanation of the origin of the median eye of the 

 Crustacea (Xo. 57), and it appears not impossible that a closer 

 ■connection may be found later between these processes. 



Further agreement between the Arachnida and the Xiphosura is 

 found in the presence of an endoskeleton, which in the Scorpiones 

 and Limulus is very similar in structure.* Another point which 

 appears to us to be very characteristic, and which also fully applies 

 to the Solifugae, in spite of their apparent deviation from the other 

 Arachnids, is the presence of a large digestive gland (liver), such as 

 does not occur in the Insecta, but is found in Limulus and the 

 ■Crustacea. Another still more important point of agreement is 

 yielded by the enteron and its appendages, if we grant that the 

 testimony of ontogeny is reliable, viz., the origin of the so-called 

 Malpighian vessels out of the entoderm. If this is the case, it 

 would form an important reason for separating the Arachnida from 

 the Insecta. Tubular appendages occur in the Crustacea at the 

 posterior end of the metenteron ; the Malpighian vessels of the 

 Myriopoda and the Insecta are, however, of ectodermal origin. 



Another point of resemblance between Limulus and the Arachnida 

 is afforded by the presence of an artery running, in the Scorpiones, 

 above the chain of ganglia, and forming a backward continuation 

 •of the oesophageal vascular ring (supra-neural vessel, supra-spinal 

 artery) ; a condition similar to this is met with in the ontogeny of 

 Limulus. A sub-neural artery, indeed, occurs in the Crustacea, 

 and a supra-neural vessel is also found in the Myriopoda (a fact 



* [There is considerable disagreement regarding the homology of the endo- 

 sternite. Bernard (App. to Lit. on Scorpiones, Xo. I.), who haa made a 

 comparative study of the Arachnidan endosternite, comes to the conclusion 

 that the endosternite of Limulus cannot be homologous with that of the 

 Arachnids, the latter being part of an epidermal endophragmal system, while 

 that of Limulus is mesodermal. On the other hand, Schimkewttsch (App. 

 to Lit. on Scorpiones, No. VI.) maintains that the structure generally termed 

 the endosternite in the Arachnida and Limulus is always mesodermal, and 

 co-exists with, but is independent of, the series of ectodermal apodemes which 

 ■are so conspicuous in Galeodes. — Ed.] 



