ON CILIARY MECHANISMS. 295 



In Crania a selection of the finer food-particles is effected in the follow- 

 ing manner : the ingoing currents are drawn into the dorsal portion of 

 the mantle cavity, hence when the dorsal valve is uppermost — as fre- 

 quently happens — the heavier particles drop on to the ventral mantle, 

 whose cilia discharges them either outside the mantle cavity or carries 

 them to the edge of the mantle, whence they are shot away by the animal 

 suddenly clapping together the valves, of the shell. Modifications in the 

 manner of rejecting unsuitable food-particles probably occur in other 

 Brachiopods in much the same way as has been observed in Lamelli- 

 branchs (1, pp. 457 to 463). 



AN EXPLANATION OF SOME MORPHOLOGICAL CHARACTERS 

 IN FOSSIL AND RECENT BRACHIOPODS. 



In the foregoing description it has been shown that in certain Brachio- 

 pods, as, for example, Crania, there is a physiological subdivision of the 

 main part of the mantle cavity into right and left portions, and it would 

 seem highly probable that in all Brachiopods there is a similar sub- 

 division of the mantle spaces, since the disposition of the lophophore is 

 fundamentally similar in them all. Unfortunately few observations 

 have been made on the nutritive currents of Brachiopods, but those of 

 Morse on Lingula (7, p. 157) are of interest. Morse found that Lingula 

 lives embedded in sand, and that " while partially buried in the sand 

 the anterior border of the pallial membranes contract in such a way as 

 to leave three large oval openings, one in the centre and one on each side. 

 The bristles, which are quite long in this region of the animal, arrange 

 themselves in such a way as to continue these openings into funnels and 

 entangle the mucus which escapes from the animal ; these funnels have 

 firm walls. A continual current is seen "passing down the side funnels and 

 escaping by the central one."* These observations have been confirmed 

 by Frangois (8) in so far as he figures the trilobed apertures of the burrows 

 of this animal (see also Camb. Nat. Hist., Vol. 3, Fig. 321). Thus there 

 can be little doubt that the mantle cavity is subdivided in Lingula in 

 the same way as in Crania. 



In these respects it is a highly interesting fact that many Brachiopods 

 both recent and fossil have a trilobed shell whose apertures correspond 

 to the inlets for ingoing currents and the outlet for the outgoing current 

 (see Fig. 8 C, p. 296), but it is probably still more interesting and remark- 

 able that in some forms, as in Conchidium Knighti, the mantle cavity is 



* The italics are mine. 



