ARANEIDA. 817 



but reduced in others. They open on the third pair of legs in 

 the young and traces of other paired openings have been found in 

 the young of Atypus on the first and second legs. 



The heart (11) is abdominal in position, dorsal and in some 

 species concealed by the " liver " tissue. It lies in a distinct 

 pericardium (22) and has three pair of ostia opening into it. 

 Posteriorly it gives off a caudal artery, laterally three pair of 

 arteries which supply the abdomen, and anteriorly an aorta 

 which gives blood to the cephalothorax. The arteries are 

 fairly well developed in spiders, but there are no capillaries, 

 and the blood ultimately finds its way into sinuses, of which 

 three chief ones in the mesosoma conduct the blood to the 

 lung-books where it is aerated. There are also three in the 

 abdomen which lead the blood in the same direction. From 

 the lung-books the blood returns to the heart by pulmonary 

 veins. 



The spinning glands lie in the abdomen and in the most com- 

 plete state consist of five kinds, (i) ampulliform (76') which open on 

 the anterior and median spinnerets, (ii) aggregate glands (17) which 

 open on the posterior spinneret, (iii) tubuliform glands (15) which 

 open on the middle and posterior spinneret, (iv) piriform glands 

 (14) which are numerous and open on the anterior and posterior 

 spinnerets, and (v) some hundred aciniform glands which open 

 on the median and posterior spinnerets. In those spiders which 

 possess a cribellum, it is supplied by a sixth set of (vi) cribellum 

 glands. These various glands supply different kinds of threads 

 which together make up the web of a spider. For instance the 

 ampulliform glands supply the first framework and the radial 

 lines of the geometric webs. The spiral lines are each 

 double and they are believed to be formed by the aciniform 

 glands, and the viscid fluid, which owing to some physical law 

 arranges itself in regular beads on them, is said to originate 

 from the aggregate glands. The tubuliform glands give rise to the 

 silk of which the egg-cocoon is made, and the pyriform glands 

 form the foundation lines and the attachment discs, which run 

 from the web to surrounding objects and serve to glue together 

 such of the non-viscid lines as the spider wishes to connect. 



A spider has apparently no power to cease secreting silk as 

 long as it is being drawn from its body, and the legs are used for 

 cutting the thread as soon as enough has emerged. The legs are 



z in 3 o 



