BLOODLESS WORMS. 77 



out any appendage (Figs. 184, 185). The dorsal side of 

 the leaf-like body differs from the ventral side, on which 

 the Worm creeps. Accordingly, even in these most simple 

 Worms there are the three definite axes which mark the 

 bilateral type-form, and which re-occur in the human 

 body and in that of all higher animals : (1) a longitudinal 

 axis (main axis), which passes from front to rear; (2) 

 a lateral axis, passing from right to left ; and (3) a 

 sagittal axis, passing from the dorsal to the ventral surface. 

 (Cf. vol. i. p. 257.) This so-called symmetrical or " bilateral" 

 arrangement of the outline of the body is simply the 

 mechanical result of adaptation to a creeping form of loco- 

 motion, during which one end of the body is always directed 

 forwards. The geometric outline of the gastrula, as of the 

 ascula, has but one axis, with unequal poles (Monaxonia 

 diplopcla). The typical outline of Worms, as of Vertebrates, 

 is, on the contrary, bilateral, with tranverse axes (Stau- 

 raxonia dipleura}. 144 



The whole outer surface of the Gliding-worms (Turbel- 

 lana) is covered, as in the gastrula, with a thick, fine 

 ciliated coat ; that is, with a fur-like covering of extremely 

 fine and close microscopic hairs, which are direct processes 

 of the uppermost cells of the epidermis, and maintain an 

 uninterrupted whirling or vibratory motion (Fig. 184, /). 

 The constant vibrations of these cilia cause a continued 

 current of water over the surface of the body. Fresh water 

 is constantly conveyed to the surface of the skin by this 

 current, thus permitting respiration in its simplest form (skin- 

 respiration). A similar ciliated covering, just as is seen in 

 the extant Gliding-worms of our fresh-water seas, pre- 

 sumably covered our extinct ancestors of the Primitive 



