THE FUNCTIONAL ORGANIZATION OF THE NERVOUS SYSTEM OF ENTEROPNEUSTA

¹ From the Department of Zoölogy, University of California, Berkeley, California

1. A picture of the functional plan of organization of the nervous system of balanoglossids, based on experiments with Saccoglossus pusillus and Balanoglossus occidentalis, is offered. The state of the receptor and effector mechanisms is also reviewed and correlated with the habit of life.

2. These animals are shown to be highly dependent on a general superficial ciliature whose complex behavior suggests the possibility of nervous control.

3. A great abundance of several types of integumentary glands capable of voluminous secretion, represents a second important effector system. Lacking any physiological evidence of the nature of the products and the variations in activity of the glands, nothing can yet be said regarding the relation of this apparatus to the nervous system.

4. Identification of the organs of light production has not been made. Crozier's argument for their independence from the nervous system is controverted but no positive statement of their nature and control can yet be made.

5. The muscular system is characterized by sluggish action and produces movements of a simple generalized character, lacking differentiation into a variety of reflexes. The paucity of clearly defined and diversified responses greatly limits the possibilities and conditions the nature of experiments on the nervous system.

6. The receptors, likewise, are at a low level of development, lower than that of many coelenterates, being represented by generalized, scattered sense cells. The mechanism of the photic response is discussed in the light of new experiments.

7. The nervous system is shown experimentally to be diffuse and superficial.

8. Properties characteristic of nerve nets—diffuse transmission, decremental conduction, and neuromuscular autonomy of every small fragment of body wall—are demonstrated to exist in balanoglossids. But the presumption of anatomical continuity of neurons from these physiological properties is not assumed, though the facts point to relations resembling those of coelenterates and differing from those of higher animals.

9. The physiological nerve net has been modified correlative with the elongated, bilaterally symmetrical body and the development of great conduction paths in the mid-dorsal and mid-ventral lines. Conduction through the general plexus is more easily induced in the transverse plane than longitudinally.

10. Specialized conduction tracts are demonstrated which correspond to the known histologic concentrations of punctate tissue.

11. Of these, the ventral tract in the trunk is less dispensable than the dorsal, but the latter may be slightly more specialized.

12. Almost no suggestions have been found that these cords, including the internal nerve cord of the collar, function in a ganglionic or central nervous capacity. They are all primarily conduction paths.

13. These facts, together with the general behavior of the animals and the high degree of autonomy of parts are considered to render inappropriate the designation "central nervous system" for the collar nerve cord or any or all of the concentrations of nerve tissue in balanoglossids.

14. The picture of the nervous mechanism of the Enteropneusta is that of an exceedingly primitively organized system. This must be emphasized, but is not considered to affect, one way or the other, the morphologic arguments concerning the relations of these animals and the chordates.