174 MANTLE REFLECTED OVER THE SHELL chap. 



opening is separated off by the concrescence of the mantle edges 

 beneath it, and we have the mantle united in two ^places, thus 

 forming three openings, the ventral of which is the opening for 

 the protrusion of the foot (Fig. 80, D). This is the case in 

 Yoldia, Leda, the majority of the Eulamellibranchiata (e.g. Lucina, 

 Cyrena, Donax, Fsaimnohia, Tellina, Venus, Cardium, Mactra), and 

 all Septibranchiata. In Chama and Tridacna the fused portions 

 of the mantle become more extended, and in Pholas, Xylophaga, 

 Teredo, Pandora, and Lyonsia this concrescence takes places over 

 the greater length of the whole mantle edge, so that the mantle 

 may be regarded as closed, with the exception of the three 

 apertures for the foot and the two siphons (Fig. 80, E). 



In certain genera there occurs, besides these three apertures, a 

 fourth, in the line of junction between the pedal and branchial 

 orifices. It appears probable that this fourth orifice (which has 

 been regarded by some as an inlet for water when the siphons are 

 retracted), stands in relation to the byssal apparatus (Fig. 80, F). 

 In Lyonsia, for instance, a thick byssus protrudes through the 

 orifice, which is large and open. In Solen, Lutraria, Glycimeris, 

 Cochlodcsma, Thracia, Aspergillum, and a few more genera, which 

 have no byssus, the orifice is very small and narrow. It is 

 possible that in these latter cases, the byssal apparatus having 

 become atrophied, the orifice has been correspondingly reduced 

 in size.^ 



Mantle Reflected over the Shell. — It is sometimes the case 

 that the mantle edges tend to double back over the external sur- 

 face of the shell, and to enclose it to a greater or less extent. 

 When this process is carried to an extreme, the edges of the 

 reflected mantle unite, and the shell becomes completely internal. 

 We see an incipient stage of this process in Cypraea and Mar- 

 ginella, where the bright polish on the surface of the shell is due 

 to the protection afforded by the lobes of the mantle. A con- 

 siderable portion of the shell of Scutus is concealed in a similar 

 way, while in Cryptochiton, Lamellaria, and Aplysia the shell is 

 more or less completely enclosed. Among Pulmonata, it is possible 

 that in forms like Vitrin^, Parmacella, Limax, and Arion, we 

 have successive stages in a process which starts with a shell com- 

 pletely external, as in Helix, and ends, not merely by enveloping the 

 shell in the mantle, but by effecting its disappearance altogether. 

 ^ Pelseneer, Comptes Bendus, ex. p. 154. 



