xii PHYLUM MOLLUSCA 735 



other families of the sub-order in the possession of a proboscis, of 

 a well-developed, but not greatly elongated, siphon, and of a short 

 foot. 



3. General Organisation. 



External Features, Symmetry, &o. — Few Gastropods make 

 an approach towards even superficial symmetry, and in cases in 

 which there is a near approximation towards such a state of 

 things, it seems clear from the results of the study of develop- 

 ment and of a comparison with allied forms that the symmetry 

 presented is not primitive, but has been secondarily acquired — 

 such symmetrical forms having been derived from unsymmetrical 

 ancestors. 



The departure from symmetry is most marked in the majority 

 of the Streptoneura. It may be said to be due to the develop- 

 ment of a protective shell composed of one piece and extensive 

 enough to be capable of enclosing all the soft parts ; and to 

 the extension of the foot on the ventral side as an elongated 

 muscular creeping organ. The development of the shell rendered 

 necessary an arrangement of the parts whereby the mantle-cavity 

 with the anus, the ctenidia and the excretory apertiires, should 

 come to be situated in the neighbourhood of the opening of 

 the shell, i.e., towards the head-end of the animal. The mantle- 

 cavity and associated parts (pallial complex, as the whole is termed) 

 had, therefore, to be shifted forward from its primitive posterior 

 position, and this was probably effected by arrest or retardation of 

 growth on one side and active extension on the other. In the 

 majority of cases it is the right side the growth of which becomes 

 retarded, and, in consequence, it is on the right side that the 

 pallial complex comes to travel forwards. The effect is as if, the 

 head retaining its symmetry, the parts between it and the anus 

 had undergone a process of rotation or torsion through about 180° 

 around a vertical axis passing in a dorso-ventral direction — 

 the direction of torsion being opposite that of the movement 

 of the hands of a watch (Fig. 619). 



With regard to the spiral form assumed by the shell in all 

 highly developed Gastropods, it can only be pointed out here that, 

 given the necessity for complete protection, compactness, and 

 a provision for continuous growth, the spirally-coiled cone is 

 the form of shell best adapted to all the conditions. A straight 

 cone, however directed, would be a great impediment to active 

 progression, and the coiling in a compact spiral would seem to be 

 the line of development best adapted to secure concentration and 

 strength. 



The rotation around a dorso-ventral axis is not the only form of 

 torsion leading to the markedly unsymmetrical disposition of parts 

 observable in most Gastropoda. There is also a process of torsion 

 VOL. I 3 A 2* 



