256 ARACHNIDA (INTRODUCTION) CHAP. 
Insecta and in the higher Crustacea.'. 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: (1.) the prosoma, (11.) the 
mesosoma, and (ili.) 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 les 
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 
Trochosa 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 Vebalia (p. 111). 
