ON CILIARY MECHANISMS. 



287 



occlusors and the " hinge." Into this sinus opens the anus. An outgoing 

 current from this part of the mantle cavity passes between the anterior 

 occkisors to join the main outgoing stream (see Fig. 2). Thus the currents 

 set up by the mantle cilia on each side of the mantle cavity divide into 

 two streams, the major one passing towards the middle of the cavity 

 and out at the front of the shell in the middle line, and a minor one pass- 

 ing between the occlusor muscles into the posterior sinus, from which 

 the outgoing stream passes between the anterior occlusors to join the 

 major outgoing stream. It will be shown later that the currents set up 

 by the mantle cilia are in the main coincident with those produced by 

 the current-producing cilia on the gill-filaments and certain cilia on the 

 lophophore. 



Fig. 3. — Lateral views of liv'ing gill-filaments of Crania (x about 56). 



A. *View of portion of a filament of the ventral series (the letter is placed near 



the distal end of the filament segment). 



B. View of portion of a filament of the dorsal series (the letter is placed near the 



proximal end of the filament segment). 



C. Arrow indicating the direction in which the lateral cilia, I.e., lash. 

 I.e. Lateral cilia which produce the main current. 



D. Arrow indicating the direction in which the frontal cilia, f.c, lash, 

 f.c. Frontal cilia which collect and transport food-particles. 



CILIAEY MECHANISMS ON THE GILL-FILAMENTS AND 



LOPHOPHORE. 



The examination of living gill-filaments of Crania and Terebratula 

 under a high power of a microscope revealed the presence of two sets 

 of ciliat which correspond in function and position to those on the 

 filaments of Amphioxus, many Molluscs, and most Ascidians. These 

 sets of cilia are well shown in the transverse sections of the filaments 



* This view would serve equally well for the tips of both kinds of filaments, 

 t Blochmann (10) has already' figured the differentiation of cilia on the filaments of 

 Crania, without, however, making any observations on their function. 



