The Internal Anatomy of Spiders 



In appearance the muscles are either colourless and trans- 

 parent, or yellowish white; and are of a soft, almost gelatinous 

 consistence. When properly treated with histological reagents, 

 and examined with a microscope of moderately high power, they 

 present numerous transverse striations, like the voluntary muscles 

 of vertebrates. Unstriped muscles are rare with arthropods; 

 certain muscles, which have been supposed to be unstriped, 

 show striations when properly treated. 



The chief muscles are attached to the inner surface of the 

 body-wall, to infoldings of it, the apodemes, and to the endo- 

 sternites described in the preceding section. There are also 

 prominent muscles attached to the 

 alimentary canal, especially to the 

 sucking stomach (Fig. 150). 



It is not within the province 

 of this book to enter into a de- 

 scription of the separate muscles; 

 there is space for only a few general 

 statements. 



The muscles of the cephalo- 

 thorax are extremely well devel- p . g ijo ^T RAM 0F A 



oped; while those of the abdomen transverse section of the 



are ereatlv reduced This is ror- cephalothorax showing 



are greatly reduced. 1 nis is cor- THE PRINCIPAL muscles 



related, to a considerable extent, (after Schimkewitsch) 



with the greater number and size 



of the appendages of the cephalothorax, the appendages of the 



abdomen, excepting the spinnerets, having been lost, and to 



the presence in the cephalothorax of a powerful sucking stomach. 



The muscles of the body-wall. — The muscles of the body-wall, 

 which form the most prominent feature of the muscular system 

 of the larvae of insects that are commonly studied, are very 

 greatly reduced in spiders. This is doubtless due to the fact that 

 the segments of the cephalothorax are fused in spiders so that 

 there can be no movements between them, and that the abdomen 

 is practically unsegmented. There are present, however, ves- 

 tiges of both circular and longitudinal "skin-muscles." 



The muscles of the appendages. — A very large proportion of 

 the muscles of the cephalothorax serve to move the appendages. 

 Some of these have their origin on the inner surface of the body- 

 wall and others, on the endosternite; in each case they extend 



143 



