xii PHYLUM MOLLUSCA 717 



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 unsyminetrical disposition of parts 

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

 around a horizontal axis which, the head and foot being regarded 

 as fixed, results in the visceral mass enclosed in the spiral shell 

 coming to occupy a more or less dorsal position with the apex 

 directed backwards and to the right. 



The shifting of the pallial complex in many Streptoneura 

 proceeds so far that the complex completely or partially passes 

 across the middle line, and the anus comes to be situated to the 

 left of the mouth. The displacement of the pallial complex 

 involves two important series of changes in the internal organs, in 

 addition to the suppression of certain structures to be referred to 

 presently. In the first place there is necessarily involved a 

 throwing of the enteric canal into a loop (Fig. 629), and in the 

 second place the long pleuro-visceral connectives (vise. com. ) 

 become twisted in such a way as to assume the form of the figure 8 

 the right connective becoming supra-intestinal and the left 

 infra-intestinal. 



Universally accompanying the process of forward displacement 

 of the pallial complex, except in Haliotis and Fissurella and allied 

 Rhipidoglossa, occurs the reduction of its paired parts. Thus in 

 all the more highly developed Streptoneura the primitively right 

 (topographically left) ctenidium alone persists, and one of the two 

 kidneys is alone fully developed and functional. 



In the Euthyneura there are distinguishable various stages in a 

 process of detorsion by which the torsion tends to be reversed and 

 the pallial complex carried back towards the posterior end along 

 the right side. The pleuro-visceral connectives lose their twisted 

 arrangement in nearly all such cases ; but there is the same 

 reduction of the paired parts of the pallial complex as in the 

 Streptoneura. 



The shell in the adult limpets (Patella and allied genera) is in 

 the form of a short cone. In most of the Gastropoda it is in the 

 shape of a spiral with the turns usually in close contact with one 

 another, the inner walls of the turns coalescing to form an axial, 

 hollow or solid column the columella. The portion of the shell 

 projecting inwards between the turns of the spiral sometimes 

 becomes absorbed. In certain cases, on the other hand, the cavity 

 of the apical portion of the spiral may be cut off from the cavity of 

 the rest of the shell by the formation of a transverse partition, the 

 animal then becoming restricted to the basal portion ; or several 

 such partitions may be formed. By far the greater number of 



