1909] Anatomy and Relationship of spiders 13 



or the axis of complex symmetry, if I may use a term accepted in 

 crystallography. The position of the synaxonial plane in coelen- 

 terates and echinodcrms is quite exact since the chief axis itself 

 is determined by the points of intersection of the radii. In 

 Arthropods in which the axis is not exactly determined by the 

 structure of the body itself, we may accept as the synaxonial 

 ])lane, the plane which divides the tergites from the sternites. 

 In the case of spiders it is the plane which separates the cepha- 

 lothorax from the sternum. Any plane parallel to this plane 

 I will call parasynaxonial. 



3. The Diaxonial Plane (usually called the transverse 

 plane) is a plane intersecting the former two (and the chief axis) 

 at right angles. Any plane which does this is a diaxonial or 

 transverse plane. 



An exact terminology may be derived from these terms for the 

 different planes, surfaces and edges of appendages, such as legs, 

 antennae, palpi, chelae, etc., since all these are built on the prin- 

 ciple of bilateral s^^mmetry. In consequence they have all three 

 determining planes of their own. Such terminology is especially 

 important in systematics. To comprehend the new terms clearly, 

 one must imagine all legs as growing out of the sides of the ani- 

 mal at a right angle to its chief axis and the chelae and mouth- 

 parts as parallel to it. (Fig. I.) For the sake of convenience 

 we may drop the prefix "para," since this does not impair clear- 

 ness. As will be easily understood from an examination of the 

 diagram, the terms apply to both sides of the body equally well, 

 so that it is not necessary to mention "left" or "right" and yet 

 no confusion can result. The same term applies to a surface 

 whether it be a leg of a mammal or of a spider, whether a front 

 or a hind leg. 



1. The Episynaxail Surface (should be epiparasynaxonial) 

 commonly called "dorsal." The term dorsal is however object- 

 ionable since the articulation of the limbs is frequently such that 

 the episynaxial surface is not really dorsal in position, as for 

 example in the crabspiders. 



2. The Hyposyxaxial Surface (should be hypoparasynax- 

 onial) commonly called "ventral," the latter name being object- 

 ionable on the same ground as "dorsal." The terms dorsal and 

 ventral I will apply to the body only, not to the appendages. 



3. The Prosymmetrical Surface (should be proparasym- 

 metrical) is the lateral surface nearest to the anterior end of the 



