174 



MANTLE REFLECTED OVER THE SHELL 



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 Eulamellibran- 

 chiata (e.g. Luciria^ Cyrena^ Donax^ Psammohia^ Tellina^ Venus, 

 Cardium^ Mactrd), and all Septibranchiata. In Ohama and Tri- 

 dacna the fused portions of the mantle become more extended, 

 and in Pholas, Xylophaga, Teredo, Pandora, and Lyonsia this 

 concrescence takes place 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, Lutra- 

 ria, G-lycimeris, Oochlodesma, 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 corre- 

 spondingly 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 inter- 

 nal. We see an incipient stage of this process in Cypraea and 

 Marginella, where the bright polish on the surface of the shell 

 is due to the protection afforded by the lobes of the mantle. A 

 considerable 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 Vitrina, Parmacella, Limax, and 

 Arion, we have successive stages in a process which starts with 

 a shell completely external, as in Helix, and ends, not merely 

 by enveloping the shell in the mantle, but by effecting its dis- 

 1 Pelseneer, Comptes lieudus, ex. p. 154. 



