648 PROFESSOR E, RAY LANKESTER. 
The generative apertures are placed far forward—ances- 
trallyin the first segment of the ‘mesosoma’ or region following 
the prosoma, and are covered by a fused pair of appendages, 
or, when these have aborted, by the corresponding sternite. 
The appendages of the mesosoma posterior to the 
generative apertures carry peculiar respiratory lamelle, 
which expose the blood circulating in them to the dis- 
solved oxygen of natural waters in the more archaic 
members of the group, but are perforated, invaginated in 
recesses of the ventral integument, and filled with atmos- 
pheric air in terrestrial forms (Scorpions, Spiders, &c.), or 
may be altogether aborted and replaced by trachee. 
Except in the Trilobita the segments and paired appendages 
of the mesosoma are not more than six in number, and the 
same is true of the metasoma or terminal region of the body, 
which is devoid of appendages (except in Trilobita), and 
may either have the appearance of a simple continuation of 
the mesosoma (macrourous forms), or may have its seg- 
ments fused with one another, but separate from those of 
the mesosoma (Trilobita) ; or, again, may be more or less 
completely aborted and fused with the mesosoma (Limulus), 
when the segmentation of the mesosoma itself may also 
become partially (Spiders) or completely (Acarina) oblite- 
erated. 
In all the larger known forms (Limulus, Scorpio, Mygale) 
a large free sclerite, the entosternite, is found within the 
prosoma, giving attachment to muscles inserted into the 
sternites of the mesosoma. 
Tabular view of the Orders of Arachnida. 
GraDE A.—H#MATOBRANCHIA (= MeRosTOMATA) : 
Orderl . f : ; : Trilobita. 
Ph Dae : : : : Eurypterina. 
Bi Aa : y ; : Xiphosura. 
GraDE B.—ABROBRANCHIA : 
Orderl . : : ; 3 Scorpionina. 
Ren D> th, ; : : : Pedipalpi. 
iS BD ves : : ? , Araneina. 
GRADE C.—LIPOBRANCHIA : 
Otter. *. . : : Solifuge. 
eee. : : f ; Pseudoscorpionina. 
ue. ; 5 : ; Opilionina. 
eae. ; : ; ; Acarina. 
