The Internal Anatomy of Spiders 



this and continuous with the hypodermis; and, bounding the 

 epithelium next to the body-cavity, a basement membrane, 

 continuous with the basement membrane of the body-wall. 

 As with insects the chitinous intima is thickened spirally. The 

 ramifications of the smaller tracheal branches appear to be much 

 less extended than are the tracheae of insects; this is probably 

 due to the book-lungs being the chief organs of respiration. 



THE CIRCULATORY SYSTEM 



The circulatory system of spiders is an incomplete one; 

 that is, for only a portion of its course does the blood flow in 

 blood-vessels; during a part of its course it enters the body- 

 cavity, as is the case with insects, where it fills the space not 

 occupied by the internal organs. The extent of the vascular 

 system, however, is much greater with spiders than it is with 

 insects. 



The heart.- — The heart resembles the heart of an insect in 

 position, being situated in the abdomen a short distance within 

 the middle line of the dorsal body-wall, and above the intestine. 

 In many spiders it is sunken to a greater or less depth in the mass 

 of alimentary tubules; it is then difficult to expose it by dissection. 

 Its position is represented in Fig. 143; and in Fig. 157 are 

 represented some details not shown on the coloured plate. In 

 this figure, a, represents the body-wall; b and c, a lobe of the ali- 

 mentary tubules, of which b is the chalky layer; d, the dorsal 

 longitudinal venous sinus; e, the pericradium, to be described 

 later;/, the pericardial cavity; g, the longitudinal muscular layer 

 of the heart; h, the annular muscles of the heart; and k, a cardiac 

 ligament. 



The wall of the heart is composed of three distinct layers: 

 an inner, very delicate, structureless intima; an intermediate 

 muscular layer; and an outer, connective tissue layer. The 

 intima is so delicate that it is usually difficult to demonstrate its 

 presence. The muscular layer consists chiefly of annular muscles; 

 but there are also longitudinal fibres. The connective tissue 

 layer is composed of slender fibres between which elongated 

 nuclei can be seen. 



The heart in spiders is a simple tube not divided into chambers 

 by valves, as is the heart of insects. 



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