The Inter nal Anatomy of Spiders 



The endosternites are termed aponeurotic plates by sorru 

 writers, who believe that they are formed by the coalescence 

 of the tendons of muscles. 



A so-called aponeurotic plate, which occurs in the pedicel 

 of the abdomen, has been described. This, however, occupies 

 a very different position from that of the endosternites, being 

 near the dorsal body-wall above the aorta. The source of this 

 plate has not been determined. 



In the sac-like portion of the abdomen, there are vestiges 

 of an endoskeleton. These vestiges are three centres of attach- 



Fig. 149. THE THREE ABDOMINAL ENDOSTERNITES AND THE 



PRINCIPAL MUSCLES ATTACHED TO THEM 



en 1, en 2, en 3, first, second, and third abdominal endosternites (after Schimkevvitsch) 



ment of muscles, one opposite the epigastric furrow, one a short 

 distance back of this, and one near the furrow of the posterior 

 spiracle (Fig. 149, en 1, en 2, en 3); these may be designated 

 as the first, second, and third abdominal endosternites respectively. 

 The endosternites of the sac-like part of the abdomen are 

 described by authors as intermediary tendons; but it seems clear 

 to me that they are serially homologous with the endosternite 

 of the cephalothorax. Correlated with the loss of the abdominal 

 appendages there has been a reduction of the connection of the 

 endosternites with the body-wall. 



THE MUSCULAR SYSTEM 



A striking feature of the muscles of spiders, and also of other 

 arthropods, is the distinctness of the muscular fibres; each muscle 

 being composed of a number of distinct, more or less isolated, 

 straight fibres; and the fibres constituting a muscle are not 

 enclosed in a common tendinous sheath, as is the case with 

 vertebrates. 



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