ON VERMICULUS LIMOSTJS. 



109 



citium, and the cell boundaries can be recognized only in the peripheral part. 

 Here and there the canal is enlarged and forms the ciliated ampullae. 



Vascular system. 



The main part are the dorsal and ventral vessels. The dorsal vessel divides 

 into two in the prostomium, and these two vessels after dividing several times 

 supply the brain and the body-wall of the most anterior part. The lateral 

 vessels, which arise from the dorsal vessels, are very small in segments I, II, and 

 III ; they gradually become larger in segments IV — X, in which last segment 

 they are considerably dilated. The largest lateral vessels lie, however, in seg- 

 ment XI, and from this on the lateral vessels are exceedingly small and just 

 recognizable. The dorsal vessel alone beats, although the ventral vessel also ex- 

 ecutes some inconstant pulsatory movements. In each segment the dorsal vessel is 

 provided with a group of valvular cells, which are also present in the constricted 

 portions of the lateral vessels. These valvular cells are more numerous in the 

 larger vessels ; they are pear-shaped and contain a granular substance. The 



Cut 5. 



D.V. Dorsal vessel, L.D.V. Lateral dorsal vessel, L.T.V. Lateral 

 ventral vessel, V. V. Ventral vessel. 



