The Internal Anatomy of Spiders 



When freshly formed, as is done before each moult, the cutio 

 ula is flexible and elastic, and certain portions of it, as at the nodes 

 of the body and of the appendages, and throughout the greater 

 part of the wall of the abdomen in spiders, remain so. But the 

 greater part of the cuticula of the cephalothorax and of the 

 appendages becomes firm and inelastic; this is probably due 

 to a chemical change resulting in the production of chitin. 

 What the nature of this change is or how it is produced is not 

 yet known; but it is evident that a change occurs; we may speak, 

 therefore, of chitinifed cuticula and of non-chitini{ed cuticula. 



The hinge-like movements of the body and of the appendages 

 are made possible by the non-chitinized condition of the cuticula 

 at the joints. 



A portion of the body-wall that is sharply distinguished 

 from surrounding portions by being chitinized is called a sclerite; 

 and a narrow line between two sclerites is termed a suture. 



I he basement membrane. — The inner ends of the hypodermal 

 cells are bounded by a very delicate, but more or less distinct, 

 membrane, the basement membrane (Fig. 144, bm). 



b. THE CUTICULAR APPENDAGES 



Under the head of cuticular appendages are included those 

 outward prolongations of the body-wall that do not form an in- 

 tegral part of it, being separated from 

 it, in each case, by a flexible joint; 

 such appendages are the hairs or 

 setae and the movable spines. 



The fixed spines, as for example 

 those borne by the abdomen of 

 Micrathena (Fig. 145), are not re- 

 garded as appendages. These form 

 an integral part of the body-wall and 

 differ only in size from mere nodules 

 on the surface of the body. 



The hairs or setcr.- — These vary 

 greatly in their external form; but 

 no distinction, except in degree of 

 stiffness, can be made between the extremely slender and very 

 flexible type commonly called hairs and the stiffer one commonly 



Fig. 145. MICRATHENA 



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