The Internal Anatomy of Spiders 



type of glandular hair that interests most the student of spiders 

 is that known as the tenent hairs. These are found on the 

 terminal segments of the legs and serve to aid in walking. For 

 figures of tenent hairs see page 124. 



C. THE ENDOSKELETON 



In the Arthropoda, as already stated, the outer covering 

 of the body serves as a skeleton as well as a protecting shield; 

 but in addition to the firm outer layer of the body-wall, there 

 are found, within the body-cavity certain hard parts which serve 

 for the attachment of muscles; these constitute the endoskeleton. 



The presence of these parts that constitute the endoskeleton 

 forms only an apparent exception to the statement that the 

 body-wall constitutes the skeleton; 

 for these parts are merely infolded 

 portions of the body-wall. 



The apodemes. — The infoldings 

 of the body-wall that serve for the 

 attachment of muscles vary greatly 

 in form; they may be merely a 

 slight ridge or they may be a prom- 

 inent projection into the body 

 cavity. In the latter case such a 

 projection is termed an apodeme. 



The endosternites. — In the ce- 

 phalothorax of spiders there is a 

 horizontal plate above the ventral 

 ganglion and below the alimentary canal, to which many muscles 

 are attached. This is known as the endojternite; but as there are 

 endosternites in the abdomen also, it may be specifically desig- 

 nated as the endosternite of the cephalothorax (Fig. 148). 



The question of the origin of this endosternite has been much 

 discussed; but it has been shown by Bernard ('96) that it is formed 

 by the fusion and expansion of the tips of four pairs of apodemes, 

 a pair extending into the body cavity in front of each pair of 

 legs. The position occupied by these apodemes is that of the 

 original intersegmental spaces, vestiges of which remain as lines 

 across the sternum in very young spiders. The endosternite 

 is, therefore, a part of the body-wall, like the more simple apodemes. 



Fig. 148. THE EXDOSTERNITE OF 

 THE CEPHALOTHORAX OF ARANEA 

 (after Schimkcwitsch) 



141 



