ON CILIARY MECHANISMS. 307 



COMPARISON OF THE ADAPTATIONS IN BRACHIOPODS AND 

 LAMELLIBRANCHS TO THE PROCESS OF FEEDING. 



The general resemblance of the shell and mantle lobe in Brachiopods 

 and Lamellibranchs, as well as the general similarity of their feeding 

 organs, renders a comparison of the adaptations in these groups to the 

 process of feeding a matter of some interest. It has been shown in an 

 earlier work (1, p. 463) that it is highly probable that Lamellibranchs 

 have evolved in the direction of perfecting the gill as a feeding organ. 

 In all Brachiopods, so far as is at present known, the feeding organ, the 

 lophophore with its gill-filaments, is in a condition comparable with that 

 of the lower Lamellibranchs, namely, the Protobranchia and the more 

 lowly Filibranchia. No process of fusion of the filaments appears to have 

 taken place in any Brachiopod similar to that generally recognised as 

 having occurred in Lamellibranchs. Since in the latter group this process 

 of fusion appears to have been one of the main factors governing the 

 evolution of that group, the absence of such a process of fusion in Brachio- 

 pods may very probably be one of the factors which has resulted in the 

 present decadent condition of that group. Along with the absence of 

 fusion of the gill-filaments in Brachiopods are correlated the absence of 

 fusion of the mantle lobes and the formation of siphons, both of which 

 characters are in many cases of prime importance in the feeding process. 

 Doubtless other disadvantages under which Brachiopods suffer — in 

 comparison with Lamellibranchs in general — are the absence of a loco- 

 motory organ in the adult stage represented by the foot in Lamelli- 

 branchs, and the sedentary life necessitated in Brachiopods by their 

 structure. These two factors debar Brachiopods from the liberty 

 Lamellibranchs mostly have of moving about, and so being able to 

 place themselves to some extent in a suitable environment. Thus, 

 therefore, probably the sum of these disadvantages may account in some 

 measure for the decadence of the group of Brachiopods, as compared 

 with the flourishing condition of Lamellibranchs at the present day. 



It has been shown that the mantle cavity in Brachiopods is physiologi- 

 cally and sometimes morphologically divided into two compartments. 

 This condition is absent in Lamellibranchs, where the mantle cavity in 

 all forms is physiologically entire. It is true that in all Lamellibranchs 

 the mantle cavity is divided either temporarily or permanently into 

 inhalent and exhalent chambers by the gill (see 1), but a corresponding 

 temporary division occurs in both compartments of the Brachiopod shell 

 when the animal is feeding. The nearest approach in Brachiopods to the 



NEW SERIES. — VOL. X. XO. 2. JUNE, 1914. U 



