124 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



of the extinct trilobites. Limulus was formerly classified 

 among the Crustacea, but is now considered to have its 

 closest affinities among the Arachnida. 



Branch XI. MOLLUSC A 



A mollusk is a soft-bodied animal, without internal 

 skeleton, and without segmentation of body or of parts, 

 covered with a moist, sensitive, contractile skin, which, 

 like a mantle, loosely envelops the creature. In some 

 cases the skin is naked, but generally it is protected by 

 a calcareous covering (shell). The length of the body 

 is less in proportion to its bulk than in other animals. 

 The lowest class has no distinct head. The nervous 

 system consists of three well-developed pairs of ganglia, 

 which are principally concentrated around the entrance 

 to the alimentary canal, forming a ring around the 

 throat. The other ganglia are, in most cases, scattered 

 irregularly through the body, and in such the body is 

 unsymmetrical (Figs. 331, 332). The digestive system 

 is greatly developed, especially the " liver," as in many 

 aquatic animals (Figs. 242, 243). Except in the cepha- 

 lopods, the muscles are attached to the skin, or shell. 

 There is a heart of two chambers (auricle and ventricle) 

 or three (two auricles and ventricle). As in all inverte- 

 brates, the heart is arterial. In mollusks, with rare 

 exceptions, we find no repetition of parts along the 

 antero-posterior axis. They are best regarded as 

 "worms" of few segments, which are fused together 

 and much developed. The total number of living species 

 probably exceeds twenty thousand. The great majority 

 are water breathers, and marine ; some are fluviatile or 

 lacustrine, and a few are terrestrial air breathers. All 

 bivalves, and nearly all univalves, are aquatic. Each 

 zone of depth in the sea has its particular species. The 

 most important classes are now to be described. 



