GENERAL CONSIDERATIONS REGARDING THE ARACHNIDA. 117 



been stated above, that there is a close relationship between the Arachnida and 

 Limulus, and, consequently, points of agreement with the Trilobites might be 

 expected. It is, in this connection, a striking fact that the Scorpiones are of 

 such great age, and that the forms now extant are not very unlike those found 

 in the Silurian strata (PalacopJwnus uuncius, Xo. 15). 



In conclusion, we must again emphasise the fact that the apparent 

 agreement of the Arachnida with the other Tracheata must he 

 regarded as nothing more than a similarity determined hy their 

 common Arthropodan nature and by a like development as the 

 result of a similar manner of life. We must not assume a nearer 

 connection between these divisions of the Arthropodan stock. We 

 believe, rather, that the Arachnida, together with the Palaeostraca, 

 proceeded from a common ancestral form, and subsequently diverged 

 from one another, while the other Tracheata belong to a distinct stock, 

 the two, however, being connected very far back. 



The Arachnida, according to our view of them, form a very 



uniform group. The most primitive forms are those in which the 



body is distinctly segmented, i.e., the Scorpiones and the Pedipalpi.* 



The Opiliones and the Pseudoscorpiones are affected by a reduction 



which goes still further in the Araneae, and reaches its highest 



degree in the Acarina, in which this far-reaching adaptation is 



accompanied by essential modifications in development.! Such 



modifications are also found in the Pseudoscorpiones, probably as 



the result of similar causes. 



[In addition to the editorial footnotes inserted here and there referring to 

 Bernard's Arachnidan work, it is necessary to call separate attention to it 

 in some detail, inasmuch as it has a profound bearing upon the question as to 

 whether the Arachnids could be deduced from a Limuloid ancestral form. 

 Arguing that the only scientific method of arriving at the ancestral form of the 

 Arachnida is to compare all the known forms, and to sift out what are obviously 

 the more primitive structural adaptations from the more specialised, this author 

 arrives at the conclusion that the Solifugae come nearest the ancestral form in 

 their segmentation, and in the simplicity of their endosternites. This endo- 

 sternite has no resemblance whatever to the endosternite of Limulus, to which 

 he would assign an entirely different origin (App. to Lit. on Arachnida in gen., 

 No. I., and App. to Lit. on Scorpiones, Nos. I., VI.). He endeavours to show that 

 the typical form of the Arachnidan body is an adaptation to the special manner 

 of feeding. The Arachnids suck the blood of their victims, and, by a force-pump 

 action of the oesophagus, distend the alimentary canal in a manner which would 

 seriously interfere with the rest of the organisation. Their whole inner anatomy, 

 he believes, can be shown to be simply so many adaptations to this serious 



* [According to Bernard, the Solifugae are in this respect the most primi- 

 tive. — Ed.] 



t [This statement is a little misleading, for, in the adult Opiliones, only six 

 segments are visible in the abdomen, while, in the Pseudoscorpiones, there are ten 

 to eleven ; further, although the abdominal somites are fused in most adult 

 Araneae (not in Liphistius), yet, in the young, eight to nine segments can be 

 recognised ; these are not lost, but fused together, and, even in the Acarina, one 

 form (Ixodes) exhibits marked segmentation ("Wagner). — Ed.] 



