VII MOLLUSGA—THE SHELL 67 



adductor, the opening of the shell, such as it is, is brought about by the muscles. 

 The anterior and upper edges of the valves are bent outward, and to these edges the 

 anterior muscle is attached. We thus have external instead of internal points 



1 

 Fio. 66.— Pholas dactylus, right valve, internal aspect (after Egger). 1-2, Axis round which the 

 valves move upon one another ; 3-4, longitudinal axis of the shell ; 5-8, line connecting the shell 

 muscles ; 6, anterior muscle ; 7, posterior muscle ; 9, rotating point of the valves ; 10, anterior and 

 upper edge of the shell, which is bent outwards, and to which the muscle 6 is attached ; 6-9, shorter 

 anterior ; 9-7, longer posterior arm of the lever. 



of attachment, and the whole shell may be compared to a double -armed lever 

 acting along the longitudinal axis of the body, its fulcrum being at the point where, 

 in other bivalves, the hinge is found. When the anterior muscle contracts, the shell 

 opens posteriorly and ventrally ; when the posterior adductor contracts, the shell 

 closes (Fig. 66). 



D. Cephalopoda. 



The Cephalopoda are all to be derived from an ancient fossil form which possessed 

 a chambered shell, in the last and largest portion of which the animal lived, leaving 

 the rest of the shell empty, or rather filled with gas (or water) and traversed by 

 the siphon or siphuncle. Such a shell is now found only in the sole living repre- 

 sentative of the Tetrabraiuhia, the Nautilus, an animal of great importance to the 

 comparative anatomist. Many fossil forms allied to the Nautilus, and grouped 

 in the order Nautiloidea, possessed such a shell, as did also the Ammonoidea, with 

 their enormous wealth of forms which, rightly or not, are generally considered to 

 be nearly related to the Nautiloidea, i.e. to belong to the Tetrabranchia. In nearly 

 all these animals the shell, when coiled at all, is, unlike the Gastropod shell, coiled 

 auterior'ly or exogastrically. 



Dne group of the Nautiloidea, the Endoceratidce, which includes only very old 

 forms (Cambrian and Lower Silurian), is distinguished by the fact that the chambers 

 of its straight shell, which were filled with gas (or water), lay at the side of and not 

 behind the inhabited chamber. There was no real siphuncle, but the upper end of 

 the visceral dome, much narrowed by the air chambers, stretched as far as to the 

 apex of the shell. 



In other Nautiloidea, ^the air chambers always lie, as in Nautilus, 'above the 

 occupied chamber, and are traversed by a thin membraneous siphuncle, which, how- 

 ever, in old forms, is much thicker, and represented the narrow prolonged portion of 

 the visceral dome (Fig. 32, p. 22). 



Some forms of Nautiloidea have shells coiled endogastrically ; this is never the 

 case, however, when the shell forms a complete spiral. The sutures, which corre- 

 spond with the lines of insertion of the septa, are simple in the Nautiloidea, as 



