62 



NERVOUS SYSTEMS AND SENSE ORGANS 



mouth. From the large ventral ganglion many branches run to 

 lateral and caudal regions of the body. This ventral ganglion is the 

 chief one from the standpoint of size. 



Many papillae on the surface of the body probably serve as 

 organs of touch. The eyes, one on each side of the dorsal region 

 of the head are globular and each contains three biconvex lenses 

 separated by pigment and surrounded by rod-like sensory cells. 

 About the dorsal part of the head end there is a ring-like ridge 

 bearing modified ciliated cells. This has been called the olfactory 

 ring. 



In Sagitta, a great proliferation of cells in the head region of 



Fig. 18. The sketch at the right is an outline of a larval Sagitta showing 

 the position of the origin of the two chief ganglia and the lateral 

 sense organs. All are indicated by the darker shaded areas. The 

 figure at the left shows the position of the chief head ganglia of 



the elongated larva forms the brain. This is added to on each side 

 by two lateral ridges which later unite to form the cephalic hood. 

 The ventral ganglion begins as a thickening of the ectoderm from 

 behind the head about two-thirds of the length of the body. A tac- 

 tile organ is developed from ectoderm on each side of the tail region 



