16 Prof. M'Intosh's Notes from the 



backward, so that an intermittent arrangement is present, a 

 feature probably due to the intervals between the thicker 

 mesenterial bands from the gut, these bands being composed 

 of fibres studded with nuclei ; and the fibres cross each other 

 on their way to those beueath the cords in the interganglionic 

 areas. The neural canal is sometimes double on one side, 

 single on the other. At the thickened perivascular areas 

 the gut touches or is sessile on the coat of the vessel. In 

 the intermediate regions, where the vessel hangs in a thin 

 mesentery, it has a pigmented coat of clavate chloragogen- 

 cells (PI. III. fig. 17, chl.), the broad end being external, so that 

 they form an arc on each side. The secretion of these, no 

 doubt, is of some importance in connection with the vascular 

 trunk and the ccelom *. Anteriorly, when the thickened 

 coat occurs, the pigmented cells are placed to the exterior 

 of the arch on the coelomic surface, but, by-and-by, in the 

 progress backward they are grouped inside the channel of 

 the tube on the blood-vessel, and this continues till it again 

 is free. The great cords are now more rounded in section, 

 with the neural canals at their upper border or at their outer 

 and upper border, and on the right side in one case two are 

 present, the larger almost extra-neural and pressing into the 

 border of the ventral muscle. Comparatively few cells occur 

 in the interganglionic areas, the general surface of the cords 

 in section being finely granular and somewhat reticulated 

 so as to form rounded areas. The cells increase at the 

 ganglionic regions, and appear chiefly in the neuroglia, only 

 a few occurring in the commissural band. Posteriorly, the 

 cords in section at a commissure are placed close together 

 with the neural canals between them, the nuclei of the 

 neuroglia scattered thinly in their area in section and more 

 thickly exteriorly. 



A short distance behind the foregoing the body-wall 

 assumes its normal arrangement, the ventral longitudinal 

 muscles lying within the hypoderm, basement-tissue, and 

 circular coat, whilst the nerve-cords and the intermediate 

 ventral blood-vessel occupy the space between their inner 

 ends. Each cord has the circular muscular coat, the base- 

 ment-tissue, and the glandular mass of the shield externally, 

 with its fibrous area inferiorly, and above it is the now large 

 neural canal, which has a firm wall and usually a coagulum 



* In a large example a peculiar and symmetrical appearance was caused 

 anteriorly by the intrusion of the massive ventral coat ef hypoderm on 

 each side of the cords and their gang-lia, so as to form an arborescent 

 mass above and on each side over the inner ends of the ventral longitudinal 

 muscles. Such probably was due to pressure in preparation. 



