ROUND WORMS 61 



hypodermis some run longitudinally in the hypodermal nerve or 

 along the sides of the body. 



There are two main types of sensory cells in the hypodermis, 

 small irregular cells staining deeply and the elongated cuticular 

 cells of the mid-ventral line. Motor cells are considered to be the 

 clearer ones of the nervous system, the darker straining cells the 

 sensory ones. These last run into the hypodermis. 



Linstrow, 1889; Ward, 1892; and Montgomery, 1897, have 

 found structures in the anterior part of the head which may be an 

 eye or possibly a part of the head ganglion. 



May, 1919, recognizes more clearly than Montgomery a ring 

 of nervous tissue in the head region. In Gordius the brain is out- 

 lined at the first as a ring of cells in the hypoderm of the proboscis. 

 It soon separates remaining connected only at the anterior end and 

 ventral side. At first it consists of a few large cells which surround 

 the larval muscles. These large cells remain in this position while 

 the rest of the brain develops in front. The ventral cord arises as 

 a thickening of the hypoderm, but later separates from it. The cells 

 that make up the nerve cord at first appear as two rows of nuclei 

 on the ventral side of the larva. The larger cells seem to be bipolar, 

 giving ofl: one fiber to the longitudinal tract and one to the dorsal 

 border of the cord. 



The brain of Paragordius develops later than that of Gordius. 

 In the first genus the cells of the lamella are located in the ventral 

 cord while in Gordius it consists of a series of cells. According to 

 May the mass of cells which Montgomery calls retina is the larger 

 pai't of the cephalic ganglion. 



The reactions of gordioid worms is slow and of a primitive 

 nature. The grasping reaction of the male when in contact with the 

 female is the most definite. If a specimen is at rest it usually re- 

 quires several successive stimuli to cause even a slight movement of 

 the body. There seems to be no distinct response to light. 



ACANTHOCEPHALIA. In this group the nervous system is found 

 to be a single ganglion of large cells located on the surface of the 

 proboscis near its base and two small ganglia in the male which 

 supply the reproductive organs. The larger cephalic center gives 

 off nerves to the proboscis in a cephalic direction and through the 

 lateral retractor muscles on each side caudally strands run out to 

 supply the body-wall. There are no sense organs known. 



Chaetognatha. In Sagitta the nervous system consists of a 

 cerebral ganglion in which eyes ai'e situated. A large ventral gang- 

 lion is situated about one-third or one-half of the way down the 

 body. Oesophageal conectives join these two chief ganglia. Fibers 

 run from the head ganglion to the jaws and sense organs of the head 

 region and two other small ganglia have been described near the 



