ON CILIARY MECHANISMS. 297 



COMPARISON OF THE CILIARY MECHANISMS ON THE GILLS 

 OF GASTROPODS, LAMELLIBRANCHS, AMPHIOXUS, AND 

 BRACHIOPODS. 



The ciliary mechanisms concerned in producing the main food and 

 respiratory current in Gastropods, Lamellibranchs, Amphioxus, Asci- 

 dians, and Brachiopods have now been shown to be essentially similar 

 in all these groups (see preceding pages and Orton, 1* and 2, and Herd- 

 man, 9). The main current is produced in all groups alike chiefly or 

 entirely by the rows of cilia, the lateral cilia situated at the sides of the 

 gill-filaments (see Fig. 9, p. 299) : for the term " gill-filament " may be 

 used as well for the gill-bars of Amphioxus or Ascidians and the lopho- 

 phoral cirri of Brachiopods as for the elements of the ctenidium of 

 Gastropods and Lamellibranchs. These rows of lateral cilia lash across 

 the length of the gill-filaments in all cases and set up a current towards 

 and at right angles to the gill. The current thus produced brings into 

 the mantle cavity — or the branchial sac in the case of the Protochordata — 

 food-particles, which serve for the nourishment of the animal, and also 

 brings the means for oxydation of the tissues, while the expulsion of the 

 current from within the cavities of the animals serves to carry away the 

 waste products resulting from the various activities of the organisms. 



The food-particles carried in the main current into the spaces of all 

 these animals are arrested on the gill which is necessarily interposed 

 between the ingoing and outgoing currents, and acts like a sieve. The 

 actual collection of food-particles varies somewhat in the different 

 groups. Food-collection is effected chiefly, however, in all the groups 

 by means of rows of cilia on those faces of the filaments facing the on- 

 coming current, that is, on the frontal faces. In some Lamellibranchs — 

 which are curiously distributed throughout the group, as Nucula, Sole- 

 nomya, Anomia, Mytilus, Cardiiim, Ostrea, Tapes — there are additional 

 food-collecting cilia on the latero-frontal edges of the gill-filaments. 

 These latero-frontal cilia are true straining cilia, and lash across the 

 length of the filament at right angles to the oncoming current and away 

 from the inter-filamentar spaces. In this way they pass particles on to 

 the frontal cilia, and indeed are so numerous in these animals as to give 



* In a paper written in 1910 Bourne (11) states of the lateral cilia of the Gastropod 

 Incisura {Scissiirella) lytteltonensis that he does not think " that their function is to hold 

 the filaments together, but simply to create currents over the surfaces of the filaments." 

 I hasten to give this reference because I only became aware of it after my paper on the 

 ciliary mechanisms in Gastropods was published. From the researches described in that 

 paper it is now seen that there is an element of truth in the suggestion made by Bourne. 



