The Internal Anatomy of Spiders 



('10 b). It differs markedly from that commonly given in the 

 text-books of zoology. 



The lateral abdominal arteries. — In the four-lunged spiders, 

 there are four pairs of lateral abdominal arteries; these extend 

 from the second, third, fifth, and sixth pairs of diverticula (Fig. 

 158, 1, la). In the two-lunged spiders, the first of these four 

 pairs is lacking (Fig. 158, 2); and in Dysdera, the first and second 

 pairs are lacking and the third is greatly reduced in size (Fig. 

 158, 3). 



The caudal artery. — The caudal artery is the backward pro- 

 longation of the heart (Fig. 158, ca). It divides into many 

 small branches, which supply blood to the spinnerets and to the 

 abdominal viscera. 



The pericardium. — At some distance from the heart, but 

 completely surrounding it is a thin-walled sac, which is termed the 

 pericardium (Fig. 158, p). The space between the pericardium 

 and the heart is the pericardial cavity (Fig. 158, pc). 



The cardiac ligaments— -The heart is supported by a con- 

 siderable number of ligaments. These extend from the heart 

 to the body-wall; some of them extend dorsally, some ventrally, 

 and some laterally. In Fig. 158, only the laterally extending 

 ligaments are represented. In Fig. 157 a dorsally extending 

 ligament (k) is shown; and several of them are represented in 

 Fig. 143. The laterally extending cardiac ligaments are termed, 

 by some writers, the alary muscles, by analogy with the "wings 

 of the heart" of insects. But Causard ('96) has shown that the 

 cardiac ligaments are not muscles, but are formed of connective 

 tissue. 



Each cardiac ligament is formed by a bundle of connective 

 tissue fibres, which is inserted in the heart, and is enclosed in 

 a tubular prolongation of the pericardium (Fig. 158, cl). The 

 cardiac ligaments that extend laterally have been designated the 

 exocardiac ligaments; those that extend dorsally, the epicardiac 

 ligaments; and those that extend ventrally, the hypocardiac 

 ligaments. 



1 he pulmonary veins. — In the basal part of the abdomen 

 of the four-lunged spiders, there are two pairs of large vessels 

 which convey the blood from the two pairs of book-lungs to the 

 pericardial cavity; and in the two-lunged spiders, there is a single 

 pair of these vessels; these are the pulmonary veins (Fig. 158, pv). 



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