388 



MOLLUSCA. 



not covered by the plates bear spicules, which are inserted in 

 ectodermal pits. 



The mantle-groove contains, in addition to the gills, the anus on 

 a short papilla in the middle line behind, and on each side the 

 opening of the kidney, and in front of that the opening of the 

 generative duct (Fig. 308). 



In the nervous system (Fig. 307) the nerve-cells are not con- 

 centrated into ganglia, but are diffused along the main nerve 

 trunks. There are two pair's of main longitudinal trunks the 

 j/eJal in the foot, and the pallial along the mantle-groove. The 

 former are connected by a number of commissures throughout 

 their extent. The latter supply the mantle, gills, and osphradia, 



E 



FIG. 300. A, Acantlwchiton fasciculuris (after Blumrich). />', Oi/yifor/ufn/i *ti II , i r<u-. 

 A (after A. v. Norclmann). C, < '/ ti^to^lax ocnlatus i (after Haddon). From Siniroth. 



and to a certain extent the viscera, and end behind by running 

 into one another dorsal to the rectum : they are to be regarded 

 as corresponding to the mantle-nerves, and partly as the much 

 extended pleural ganglia of other forms. These two cords on each 

 side join in front, and the single ganglionic cord passes round the 

 oesophagus to unite with its fellow across the middle line dorsal 

 to the oesophagus. This broad ganglionic band corresponds generally 

 to the cerebral ganglia of other forms. 



The visceral commissure (Fig. 307, VII) is given off from the 

 pallial nerves close to where these join the pedal cords, and passes 

 only a short distance backwards ventral to the alimentary canal. 



