ON CILIAKY MECHANISMS. 



285 



broiiglit along with the current by means of a suitable organ, namely 

 the lophophore, placed between the ingoing and the outgoing currents. 



The main food and respiratory stream in Crania enters the mantle 

 cavity at both sides and is expelled in the middle of the front of the shell 

 opposite the hinge (see Fig. 1). It will be remembered that the lopho- 

 phore in Brachiopods is distributed symmetrically on each side of the 

 antero-frontal axis of the shell. The effect of this disposition is — as will 

 be shown later — to divide the mantle cavity in a physiological sense* 



ingoing 

 cuTTeni 



outgoing 

 currenf 



ingoing 

 current 



Fig. 1. — View of Crania attached to a stone in the act of feeding. Drawn from the 

 living animal and chiefly from the right side ( X 10). An ingoing current is drawn 

 into the lower portion of the mantle cavity on each side in the antero-Iateral 

 region. The outgoing current leaves the region of the shell in the front middle 

 line. The double row of the protruded gill-filaments is well shown. 



into two compartments, each containing a half of the lophophore on each 

 side of the mouth. Consequently two inhalent streams are necessary and 

 enter the mantle cavity, as shown in Figs. 1 and 2. The exhalent streams 

 are, however, combined in Crania, Lingula, and probably most recent 

 Brachiopods in the middle line in the front region of the shell. 



These main food-currents in Brachiopods are produced chiefly by the 

 lashings of definite rows of cilia situated along the sides of the filaments 

 of the lophophore (see Figs. 3 and 7, pp. 287, 292). Groups of other 

 cilia, however, on other parts of the lophophore and on the mantle 



* The mantle cavity is in many genera partially divided also morphologically by 

 septa, viz. Waldheimia, Stringocephaius. (See also p. 295.) 



