40 NERVOUS SYSTEM AND SENSE ORGANS 



always as well developed and may not always be functional. In 

 this form the eyes appear as four small pigment spots partly im- 

 bedded in the dorsal surface of the brain. In this and in Tris- 

 tomum, each eye-spot consists of an almost spherical, highly refrac- 

 tive transparent body which in many cases contains one or two 

 small vacuoles, but a nucleus was not seen. The lens is partly 

 covered by a cup of dark brown pigment granules. These parts are 

 imbedded in a rather large ganglion cell. Two or three fibers arise 

 from each ganglion cell and extend some distance into the brain. 

 A series of delicate muscles are near the eyes and their contractions 

 bring about rotations of the eyes. One pair of eyes has been found 

 to move simultaneously with the other, although this does not always 

 take place. If the animals are vigorous the movements of the eyes 

 may take place with the rapidity of a heart beat. 



The eyes are situated on the dorsal side of the brain. The 

 tissue between them and the ventral side is clear and light passing 

 under the host must strike the lens and affect the retina as the 

 pigment is placed in the most favorable position in the anterior side 

 of the lens. 



In some digenetic trematodes the nei'vous system has a rather 

 complicated system of branching as shown in Amphistomum by 

 Loss, 1892. Nerve tracts are clearly defined and nerve cells, 

 although chiefly centered in the broad brain, are also found out 

 along the peripheral nerves. 



Faust, 1918, has studied the eyes in digenetic trematodes. In 

 twenty-eight species, seven possess pigmented eyes and four non- 

 pigmented ones. Binoculate species usually have the eye spots in 

 direct connection with the posterior dorsal nerve trunks. In one at 

 least connections were with the anterior dorsal rami. The central 

 eye of trioculate species is fused to the anterior dorsal nerve trunk 

 by a blunt fiber from below. The eye spots consist of a cluster of 

 rather dark-brown granules forming a deep cup. Within the cup 

 is a spherical body barely touching the pigment granules. This is 

 the enlarged nerve ending with a nucleus within. 



The development of the eyes in Cercaria gigas is as follows : 



A branch of the posterior dorsal nerve with a single nucleus 

 pushes out from the nerve center to the dorsal margin of the 

 embryo. As it reaches a position near the surface, the ectodermal 

 layer of the embryo pushes inwards just posterior to the nerve, so 

 that a pocket is formed with the opening opposed to the nerve cell. 

 The end of the nerve fiber enlarges and twists about the inner wall 

 of the pocket so that the end with the nucleus comes to lie within 

 the cup. At first the ectodermal cells are evident, but later they dis- 

 appear. Pigment granules are not present until the nerve ending 

 comes to occupy its position within the pocket. Golden-brown pig- 

 ment granules come to be formed between the nerve endings and the 



