ON THE STRUCTURE OF J!:NIGMA ^NIGMATICA. 263 



from the riglit side is clearly shown in fig-. 1. They are 

 relatively even larger than in Anemia^ and, as in that genus, 

 their posterior ends are recurved and form the posterior 

 boundary of the dorsal or supra-branchial chamber into which 

 the anus opens, and in which the ventricle of the heart lies. 

 In their microscopic as well as their macroscopic structure 

 the gills of Enigma are singularly like those of Anomia 

 ephippium as described by Ridewood (15), to whose paper 

 the reader is referred for details. Thus the reflected filaments 

 of the right and left internal demibranchs are fused together 

 and the whole series of filaments are in organic continuity 

 along the line of union, and are traversed by a blood-vessel. 

 The upper ends of the reflected filaments of the right and 

 left external demibranchs are secondarily reflected downwards 

 to form the so-called velar fold or flap, and at the angle of 

 reflection the whole series of filaments are united in organic 

 continuity, and are traversed from end to end by a blood- 

 vessel (text-figure 2,h.v.). The lower ends of the velar fila- 

 ments are, however, independent, and have an arrano-ement 

 of cilia which appears to have been overlooked by previous 

 observers. As is shown in fig. 6 these velar filaments are 

 sub-triangular in section, and the chitinous lining of their 

 cavities is thickest on the internal or morphologically ventral 

 side, this being tlie reverse of what obtains in the direct and 

 reflected limbs of the filaments. The outer (morphologically 

 dorsal) side of the velar filament is broad and flat, and is 

 covered by a cubical epithelium bearing a number of short 

 stiff cilia. The opposite face of the filament is narrowed and 

 covered by longer columnar epithelial cells bearing long fine 

 cilia, continuous with the frontal cilia of the reflected or 

 ascending limb of the filament. There can be little doubt 

 that the short stiff cilia borne on the pallial faces of the velar 

 filaments have the function of ciliated discs, and give a suffi- 

 cient amount of friction against the inner surface of the 

 mantle to prevent the whole of the reflected lamella from 

 slipping down. One might expect to find a corresponding 

 ciliated ridge, or row of ciliated discs, on the mantle, but I 



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