THE NERVOUS SYSTEM IN THE LOWER ANIMALS. 21/ 



lated Ringed Worms (Annelida) and the Star-animals {Echi- 

 noderma), which originated from Artliropods. The Soft- 

 bodied Animals {MoUusca) also have a throat ring, which is 

 quite unrepresented in Vertebrates. Only in Vertebrates 

 the central marrow developed along the dorsal side, while 

 in all other animals which have been named it developed 

 along the ventral side of the body.^"* 



Descending below the Worms we find very many 

 animals which are entirely without a nerve-system, and in 

 which the functions of that system are performed simply by 

 the outer skin-covering — by the cells of the skin-layer, or 

 exoderm. This is the ease in many low Plant Animals 

 (Zoophyta), for instance, in all Sponges, and in the common 

 fresh-water Polyp, the Hydra. It was also undoubtedly the 

 case in all extinct Gastrseads. In aU Primitive Animals 

 (Protozoa) the nerve-system is, of course, unrepresented, for 

 these have not as yet attained to the development of germ- 

 layers. 



In considering the individual development of the nerve- 

 system in the human embryo, we must first of all start from 

 the important fact already mentioned, that the first rudi- 

 ment of the system is the simple meduUary tube, which 

 detaches itself from the outer germ-layer along the middle 

 line of the lyre-shaped primitive germ. We found (Figs. 

 85-87, voL i. p. 298) that the rectilineal primitive groove, or 

 dorsal fun-ow, first arises in the centre of the lyre-shaped 

 o-erm-disc. On each side of this rise the two parallel dorsal 

 or medullary swellings. The free margins of these bend to- 

 wards each other, coalesce, and form the closed medullary tube 

 (Figs. 88-93, vol. i. pp. 300-309). At first this tube lies directly 

 under the horn-plate; it is, however, afterwards situate 



