255] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 61 



It thus lies near the middle of the head, between the main muscle strands and 

 the eyes. There is a slight division into lobes which gives the appearance, 

 at least, of a double structure, but as there is no suggestion of symmetrically 

 arranged nerve branches or in fact no regularity of position or size of fiber, 

 there is little to show a truly paired condition. The nerve cells are fairly large 

 but show little detail of development. The gangHon contains, also, other cells, 

 either for nourishment or support. These may be distinguished by the fact 

 that they have no branches at all. It may, of course, be true that they later 

 develop as true nerve cells. The nerve connections can hardly be said to be 

 definite trunks or even special nerves since they seem almost like projections 

 of the gangHon, amoeboid in their promiscuous wandering. Most of the 

 fibers, or nerves, as they may be called provisionally, find their way to the 

 skin, spreading out over the inner surface and forming a layer of receptive 

 material. There are very few nerve strands extending toward the intestine 

 and even the muscular system is poorly supplied. The greater part of the nerve 

 mass is evidently sensory, and it seems the function of the motor nerves must 

 be provided in some other way, perhaps by the direct responsiveness of the 

 muscular tissue itself. Since there seem to be no cells purely associative in 

 their activity, the various stimuli are probably passed from cell to cell. But 

 however primitive the nerve tissue may appear, it very truly shows a great 

 advance over that found in related famihes. 



Closely connected with the nervous system and acting almost as a part of 

 it, are three t)Apes of special sense organs namely: the skin, the few specialized 

 tactile or taste cell groups, and the eyes. The skin seems highly receptive 

 thruout its entire surface and is evidently the most important medium of com- 

 munication wdth surrounding conditions. Only one ceU in thickness, it serves 

 as a most efficient conductor of stimuli of probably all general kinds. That 

 it is extremely sensitive is apparent from the great number of contractions 

 constantly taking place. On the ventral surface of the head there are several 

 special groups or patches of more highly differentiated epidermal ceUs. These 

 patches are each supplied with a special nerve branch and seem to be of value 

 in selecting food and in ascertaining more exact details as to environment so 

 that they may be primitive taste or rather olfactory organs. Each is com- 

 posed of a cluster of several cells with rather heavy, rigid walls and denser 

 cytoplasm. The whole head is generally in rather constant motion, parts 

 are protruded or pushed around in all directions as tho investigating and lo- 

 cating any nearby source of food. It is with these sensory patches that such 

 activity is carried on. 



The most conspicuous development in the way of a sense organ is the eye. 

 This is hardly more than a light-detecting organ, but is noteworthy because 

 of its characteristic shape and color. The eyes lie one on each side of the middle 

 line just dorsal to the brain gangHon beneath the epidermis. The carmine- 

 red color of the pigmented portion makes them stand out sharply against the 



