XII PHYLUM MOLLUSCA 737 



presently. In the first placed there is necessarily involved a 

 throwing of the enteric canal into a loop (Fig. 619), 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 palhal 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 

 nephridia 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. 



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 por- 

 tion of the shell projecting inwards 

 between the turns of the spiral some- 

 times 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 Fm. 620. —sheii of solarium per- 

 becoming restricted to the basal IT^S^^i^'S^'^ri^l s:utS^i 

 portion ; or several such partitions Hisiori/.) 

 may be formed. By far the greater 



number of such spiral shells are dextral, i.e. if we begin at the 

 apex of the spiral, to reach the opening of the shell we have 

 to pass from left to right, with the columella always on our right- 

 hand side : in a few cases, however, the spiral is sinistral, taking 

 the opposite direction from that of the ordinary dextral shell. 

 The form of the shell varies with the degree of obliquity with 

 which the whorls are set on the axis. When the obliquity is very 

 slight (Fig. 620) the spiral is nearly flat ; when the obliquity is 

 great, an elongated tapeiing shell such as that represented in 

 Fig. 621 is the result. Sometimes the later whorls completely 

 cover over the earlier ones, so that the spiral form of the shell 



