COMPAEATIYE MOEPHOLOGT OF THE GALEODID.E. 319 



II. The Internal Apodematous Skeletal System. 



The skeleton of the segmented animals was primitively limited to the external cuticle 

 -which formed Avhat is called an exoskeleton. In specialized forms, the modiiications of 

 the sgements have, in many cases, given rise to infoldings of this exoskeleton, so that a 

 kind of endoskeleton has teen developed. Such an endoskeleton, or system of apodemes, 

 attains a remarkable and varied development in the cephalothoracic region of the 

 Arachnids, i. e. in that region specialized for locomotion where the muscles are most 

 highly developed. The posterior infoldings, viz., those which limit the closely knit 

 locomotory region posteriorly, together form the waist, which need not be again 

 described. The complicated infoldings between the six segments anterior to this wrist, 

 Tiowever, claim attention. 



The Ventral System of Apodemes. — T^"e find in the Galeodidae a ventral system of 

 apodemes forming a nearly rigid framework, or, rather, two frameworks joined by a 

 flexible membrane between the 4th and 5th segments. These frameworks are largely 

 formed by the coxae of the limbs, which meet in the middle line and practically form 

 the ventral surface, having apparently obliterated nearly all traces of the original sternal 

 surface (see above, p. 311). Perhaps the flat keel thrown up into the body along tbe 

 middle line may represent the remains of the sterna. Figures of this complicated inner 

 surface have been given by both Kittary and Blanchard, but the details are obscured by 

 the endosternite (/. e. by the largest pair of apodemes between the 3rd and 4th segments) 

 which these authors have drawn in position. PI. XXVIII. fig. 15 gives a view of 

 the whole ventral surface of a Galeodes orabs, with the endosternite removed but placed 

 at the side (fig. 15 a). 



This ventral skeletal framework is, as stated, due to the rigidity of thecoxal joints and 

 to their meeting in the middle line ; it needs no special notice. Certain modifications 

 of the transverse ridges will be mentioned below. One pair of infoldings, however, 

 require special mention. 



The Ejidostermte. — This characteristic Arachnidan structure rises as a pair of simple 

 infoldings of the cuticle between the 3rd and 4th segments (PI. XXVIII. fig. 17). The 

 two cuticular folds meet and interlock in the middle line (fig. 18) above the nerve-cords 

 and beneath the alimentary canal (PL XXVII. fig. 12). Sloping backward from their 

 origin, after meeting, they expand into plates to allow of increased surface for muscular 

 attachment. Where the apodemes arise from the cuticle, they are very hard and thick, 

 breaking and tearing the sections, but in the expanded part they are softer, and offer 

 l)ut little resistance to the microtome razor. Histologically, they agree entirely with 

 tbe outer cuticle, of which they are obviously infoldings. 



Tlie position of tlie apodemes forming the endosternite behind tbe 3rd segment, 

 which is much compressed, is what one might expect. The three anterior segments have 

 become firmly fused together, probably in the process of forcing the limbs of the fii\st 

 two segments into positions in front of and at the sides of the mouth. In this process 

 the 3rd segment seems to have suffered most compression. But compression of a 

 jsegment natiu'ally means, as a rule, a strong muscular strain on the constriction 



