256 ARACHNIDA (INTRODUCTION) CHAP. 



Insecta and in the higher Crustacea. 1 The significance of this 

 fact is not perhaps apparent, but it seems to indicate " a sort of 

 general oneness, if I may be allowed to use so strong an ex- 

 pression," as Mr. Curdle said when discussing the unities of the 

 drama with Nicholas Nickleby. 



These segments are arranged in higher categories or " tagmata," 

 of which we can recognise three : (i.) the prosoma, (ii.) the 

 mesosoma, and (iii.) the metasoma. The prosoma, sometimes 

 termed the " cephalothorax," includes all the segments in front 

 of the genital pore. According to this definition the prosoma 

 includes the segment which bears the chilaria in Limulus (the 

 King-crab) and the pregenital but evanescent segment in 

 Scorpions. The mesosoma begins with the segment bearing the 

 genital pore, and ends with the last segment which bears free 

 appendages, six segments in all. The metasoma also consists of 

 six segments which have no appendages ; together with the 

 mesosoma it forms the abdomen of some writers. The anus lies 

 posteriorly on the last segment, and behind it comes in the 

 higher forms a post-anal " telson," taking in Scorpions the form 

 of the sting, in King-crabs that of the spine. 



As we have seen, it is only in the more typical and perhaps 

 higher forms that we can find our twenty-one segments, and 

 then they are never present all at once. In many groups of 

 Arachnids the number is reduced at the hinder end, and obscured 

 by the fusion of neighbouring segments. Also segments are 

 dropped as a stitch is dropped when knitting ; for instance, in 

 the rostral segment which has a neuromere, and in the Spider 

 Trockosa vestigial antennae, or in Scorpions the pre- genital 

 segment. 



Primitive Arachnids appear to have lived in the sea and to 

 have breathed by gill-books borne on appendages ; when their 

 descendants took to living on land and to breathing air instead 

 of water, the gill-books sank into the body and became lung- 

 books, to which the air was admitted by slit-like stigmata. In 

 other terrestrial forms the lung-books are replaced by tracheae 

 which in their structure and arrangement resemble those of 

 Peripatus rather than those of the Insecta. The circulation, as 



1 This can be maintained in the Crustacea by counting the seventh abdominal 

 segment, which appears in Gnathophausia ; but this is not universally regarded 

 as a true segment. See also Nebalia (p. 111). 



