290 J. H. ORTON, 



When Crania is feeding the gill-filaments are extended in the plane 

 of the lophophore and extruded beyond the opening of the shell, as 

 •shown in Fig. 1 ; they are also usually kept close to the upper valve. 

 This disposition of the filaments permits a selection of the finer food- 

 particles, for the heavier particles in the ingoing current drop out of the 

 stream at the entrance to the mantle cavity and are rejected by the cilia 

 on the lower mantle as shown in Fig. 2. The main current, however, 

 passes between the filaments on the first turn of the lophophore and 

 onwards through the filaments of the succeeding turns of the spiral lopho- 

 phore — ^which are superposed one above the other (see Fig. 4, p. 288) — 

 to be passed out of the mantle cavity finally in the anterior middle 

 portion as has been seen. Thus in traversing the mantle cavity the 

 food-stream passes successively through a number of sieves which are 

 formed by the tiers of filaments on the lophophoral spiral. This arrange- 

 ment is probably very effective in sieving off the food-particles, although 

 at first sight the somewhat straggling appearance of the filaments does 

 not give this impression. The efficiency of the sieving action of the 

 filaments, moreover, is further enhanced by the fact that the filaments 

 are arranged in two distinct alternating rows at the edge of the lopho- 

 phore (see Fig. 5, p. 289), one row being slightly behind the other, as 

 shown in Fig. 7, which is a transverse section of a group of filaments 

 taken near their origin from the lophophore.* As a result of this dis- 

 position the lateral rows of cilia near the base of the upper filaments lash 

 directly on to the frontal face of the lower ones, that is, directly on to the 

 frontal cilia, which are the principal agents in catching food-particles. 



An additional minor factor in producing the main food and respira- 

 tory stream are the cilia on the body of the lophophore, which have also 

 been figured by Blochmann. These cilia also assist partially in collecting 

 food-particles and their action may best be gathered from Fig. 5, p. 289 

 and a diagram of a transverse section of the lophophore (see Fig. 6, p. 291). 

 The cilia on the frontal surface of the lophophore are differentiated into 

 two sets separated by a ridge running along the middle of the spiral, 

 but slightly towards the outer, Le. filamentar side. The cilia on the 

 filament ar side of this ridge are very powerful and lash towards the 

 filaments, those on the other side lash away from the ridge in a direction 

 mainly transverse to the length of the lophophore, but also slightly 

 oblique, towards the proximal end, as shown in Fig. 5, p. 289. The 



* The arrangement of the filaments on the lophophore is probably universal in 

 Brachiopods, as it has been observed by Van Bemmelen in Terebratula (6) and Bloch- 

 mann (10) in Crania, Discina, and Lingula. Oehlert (14) also makes the general statement 

 that they are arranged in two rows in Brachiopods. 



