30 WRIGHT AND MACALLUM. [Vol. I. 



The Nervous System. 



We have found Sphyranura particularly favorable for the 

 study of the nervous system during life. In no other Trema- 

 tode have we seen the fibrillation of the plasma of the ganglion- 

 cells so distinctly as in this ; and as it is easy to study living 

 young examples under one-eighth inch or even one-twelfth 

 inch oil-immersions, the lateral nerves, their commissures and 

 branches can be followed with care. 



Fig. 7 is intended to represent, diagramatically, the general 

 arrangement of the nervous system. The ganglion-cells are 

 disposed in two masses, which lie right and left of the muscular 

 pharynx, but extend both in front of and behind that organ. 

 It is to be noted that they are not grouped around the pharynx, 

 but lie well to its sides, none being found on the dorsal or 

 ventral aspects of that organ. Single cells straggling out of 

 the ganglionic area are to be found in the upper lip, and isolated 

 cells are applied to the walls of the contractile vesicles and 

 receptacula seminis, but we have found none behind this Hne. 



The lateral ganglia are connected by two commissures, the 

 stouter of which is supra-pharyngeal, and connects the ganglia 

 near the middle of their lengths, while the slenderer is infra- 

 pharyngeal and crosses the anterior arch of the intestine, con- 

 necting the ganglia at the point of origin of the lateral nerve- 

 stems. These are four in number, two to each side, and may 

 be described as lateral and ventro-lateral. Their position rela- 

 tive to each other and to the organs of the body may be best 

 gathered from Fig. 14. Of the two the lateral nerve-cord is 

 the stouter, and is more directly connected with the lateral 

 ganglion, while the ventro-lateral receives many of its fibres 

 from the infra-pharyngeal commissure.^ 



The transparency of the body of Distomum isostomum, 

 enabled Gaffron^ to detect an arrangement of the nerve-cords, 

 and commissures much more complicated than had been 

 supposed to occur in Trematodes. He describes six nerve- 

 stems, four of which agree in position with those referred to 

 above, but two of which, dorsal in position, are unrepresented 

 in Sphyranura. The ventral nerves unite posteriorly, as do the 



»Zo61. Anz., VI,, 508. 



