200 



THE INVERTEBRATA 



here takes the form of a cerebral gangHon is best regarded as a 

 development in connection with the special sense organs, from which 

 it receives stimuli. The cerebral ganglion functions as a relay system 

 in which the stimuli received from the special sense organs are re- 

 inforced, often extended in time, and then passed on to the nerve 

 net. When this sensory relay has been destroyed by removing the 

 cerebral ganglia, the nerve net is no longer excited to bring the 

 muscular system into action, although this may still be done by 

 artificial stimuli. 



Sense organs occur in adults only in the free-living Turbellaria, 

 where they may take the form of eyes, otocysts, tentacles and ciliated 



CB <^ \ <Si) (S) €D ~--ect. 



-pt-c. 



-nu. 



Fig. 146. Eye of Planaria luguhris. From Hesse Doflein. ect. ectoderm; 

 nu. nucleus of pigment cell; pi.c. cup-shaped pigment cell forming retina; 

 sp.c. special light-sensory nerve cells with fibrillae {ff.) extending to retina. 

 Arrow indicates line of vision. 



pits in the ectoderm. They may also occur in the free stages in the life 

 history of the Trematoda and Cestoda. The eyes occur on the dorsal 

 surface where they are visible as dark spots. The retina is formed of 

 cup-shaped cells, which are heavily pigmented. The interior of the 

 cup is filled with special nerve cells, varying in number from two 

 to thirty, the fibrillae of which touch the retina, and the fibres at the 

 other end are joined together to form an optic nerve leading to the 

 brain. There is no lens, but the ectoderm over the eye is not pig- 

 mented and so permits light to pass through it (Fig. 146). It should 

 be noted that in this simple eye, as in the extremely complicated organ 

 found in the vertebrates, the light has to pass through the sensory 

 cells of the nervous system before it reaches the retina, for they are in 



