176 



THE INVERTEBRATA 



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. 142). 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 

 front of, not behind, the retina. This type of eye is easily seen and 

 studied in the common freshwater planarians. In Planaria torva, the 

 eye has only two sight cells, while in Planaria lactea there are thirty. 

 Special sensory cells, which differ from the ordinary ectoderm cells, 

 and which are directly connected with the underlying nervous net- 



m <S) 



<m) 



(B 



<2) ---ect. 



Fig. 142. Eye of Planaria torva. 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 (j^*".) extending to retina. 

 Arrow indicates line of vision, 



work, occur on the dorsal surface. They are usually provided with 

 long cilia and may be sunk into pits, which are called ciliated pits (see 



Fig. 145)- 



The tentacles are projections of the body wall near the anterior end. 

 They are found in the Turbellariaonly, but are not present in all these. 

 When present they are quite distinct and have very long cilia which, 

 by their motion, set up currents and so lead us to suppose that their 

 use is for water-testing, or searching for food. Occasionally these 

 tentacles may be sunk into pits. 



A statocyst occurs in primitive forms of the Turbellaria and is of 

 the otocyst type. It is situated above the brain and suggests a con- 

 nection with the Coelenterata where such sense organs are common, 



