ON CILIARY MECHANISMS. 309 



duced chiefly by rows of lateral cilia on the cirri, or gill-filaments, but 

 cilia on the body of the lophophore and on the mantle assist in main- 

 taining this current, which in Crania enters the mantle cavity in the 

 antero-lateral regions of the shell at each side and is expelled in the middle 

 front part of the shell. The mantle cavity in Brachiopods is divided 

 physiologically into two compartments corresponding to the bilateral 

 symmetry of the lophophoral spirals. In some Brachiopods the mantle 

 cavity is also divided morphologically into two compartments, either by 

 medium antero-posterior septa or by actual bifurcation of the whole shell. 



Food-collection is effected in Brachiopods mainly by the frontal 

 cilia on the gill-filaments, but tracts of cilia on the filamentar side of the 

 lophophore assist in capturing food-particles. The captured food from 

 both these sources is passed into the well-known food-groove on the 

 lophophore and thence to the mouth. 



For the capture of food-particles mucus is secreted on the frontal 

 epithelium of the gill-filaments, at the bases of the gill-filaments, and on 

 the body of the lophophore. A selection of the finer food-particles is 

 effected in Brachiopods to some extent in the lower parts (inhalent 

 chambers) of the mantle cavity ; the heavier undesirable particles being 

 collected on the mantle lobe from which they are expelled from the pre- 

 cincts of the animal. 



Some of the characters of the shells of many fossil and recent Brachio- 

 pods can be partially explained from the fact of the physiological sub- 

 division of the mantle cavity. 



The cephalic gills of Cryptocephalous Polychsetes have current-pro- 

 ducing lateral cilia and frontal food-collecting cilia essentially similar to 

 those of Brachiopods, Lamellibranchs, and the other groups mentioned 

 above. In Phoronis there is also a similar differentiation of cilia on the 

 lophophoral cilia, but so far as this animal has been examined the func- 

 tions do not appear to be quite the same as in the other groups 

 examined. 



Thus the ciliary mechanisms on the gills of many Gastropods, most 

 Lamellibranchs, Amphioxus, Ascidians, Brachiopods, and the Crypto- 

 cephalous Polychsetes are essentially similar, and in the three groups to 

 which these animals belong the same mechanism — ^and similar gill-filament 

 supports — have arisen independently to meet similar or the same require- 

 ments, and thus afford an interesting example of parallel evolution. 



An endostyle is present at the base of the gill in Crepidula, Calyptrsea, 

 and probably also in Capulus. This endostyle of Crepidula shows a 

 remarkable resemblance to that of Amphioxus and Ascidians and 



