298 J. H. ORTON. 



the appearance of flexible combs working along the sides of the filaments. 

 The frontal cilia in all forms lash the food-particles into a food-groove, 

 whence the particles are carried to the mouth or oesophagus. It is an 

 interesting fact that the frontal cilia are in all cases short, as compared 

 with the length of the lateral or the latero-frontal cilia, and it is probable 

 that short cilia would be much more effective than long ones in trans- 

 porting food-particles and masses of particles embedded in mucus, and 

 would, moreover, be more easily controlled and less liable to become 

 intermixed and so interfere with their fellows. 



Food-collection is also assisted in some Gastropods as in Crepidula, 

 and some Lamellibranchs as in Nucula, by rows of cilia on the ab-frontal 

 faces of the filaments, but probably the function of these rows of cilia in 

 assisting in producing the main current is more important than that of 

 food-collecting. In Amphioxus and Ascidians cilia on the epithelium of 

 the atrial cavity help in a small way in producing the main stream. 



The distribution of the cilia on the gill-filaments of Gastropods, 

 Lamellibranchs, Amphioxus, and Brachiopods is shown in the 

 transverse sections depicted in Fig. 9, p. 299. From these figures the 

 essential similarity of all the filaments is at once apparent. The position 

 of the lateral cilia is, however, somewhat different in the types of filament 

 represented by those of Crepidula and Crania. In those filaments the 

 lateral cilia occupy a position much nearer the ab-frontal surface of the 

 filament. It is probably significant that in both Crepidula and Crania 

 the gill-filaments are free and without any extensive interlocking arrange- 

 ments such as exist in Lamellibranchs, or such a consolidation as occurs 

 in Amphioxus and Ascidians. It is very probable therefore that the 

 receding of the lateral cilia from the frontal surface in these forms is an 

 effort to compensate for the lack of compactness in the gill, by exposing 

 a larger food-collecting, that is, frontal surface. 



A glance again at Fig. 9 shows that iii all these filaments internal sup- 

 ports (g.s.) are developed similarly, but with some differences in order to 

 maintain the gills sufficiently rigid in the form of either an open basket- 

 work or grid-iron sieve. 



In an earlier paper (2) it has been suggested that the function of the 

 gill in Crepidula, most Lamellibranchs and Amphioxus is merely that of 

 a water-pump and a food-sieve, and that the respiratory function is not 

 performed to any appreciable extent on the gill in these forms. In 

 Brachiopods, Shipley (4, p. 501) has arrived at the same conclusion with 

 regard to the function of the lophophore, of which he states : " I have 

 been unable to detect any blood corpuscles in the tentacles, and I believe 



