138 PRIMITIVE FORMS OF LIFE 



reaction of the water thus expelled pushes it forward, and the 

 resistance of the sea-water again intervenes to force backwards 

 the point of the cone, which is then pulled downwards by 

 gravity. All these co-ordinated actions, added to the phenomena 

 of growth, inevitably determine the coiling of the shell in a 

 spiral form, the opening being directed backward and the 

 coiled part forward, as is seen in Nautilus. 



This is also the form of coiling found in the young shell of 

 the oldest Gastropods (species of Fissurella, Trochus, etc.), from 

 which we may conclude that their gills were primitively posterior. 

 These Gasteropods, like the Cephalopods (Bellerophon), were 

 originally swimming organisms, and were also obliged to coil 

 their shell forwards. The pressure of the water against the 

 shell carried thus forwards and spirally wound sufficed then to 

 keep the posterior branchial aperture open. Later, however, these 

 molluscs become crawlers (species of Pleurotomariidse, Fissurel- 

 lidae, Haliotidse, Trochidae, Turbo, etc.), and once more had to 

 apply the ventral aspect to the ground surface. The forward- 

 directed shell then became directed backwards, as a result of 

 crawling, and again masked the branchial aperture. Lang 1 has 

 shown how the Gasteropods got out of the difficulty by con- 

 tracting one half of the body, so as to turn the branchial 

 cavity to the front, and Robert 2 has been able to trace the 

 phases of this rotation in the larvae of Trochus. The almost 

 permanent contraction of one half of the body has gradually 

 induced a shortening and then a partial abortion of this half ; 

 the spiral coiling of the cone thus became dissymmetrical, and 

 was replaced by a corkscrew formation. The torsion into a 

 figure 8 of the nerve cord, from which the visceral nerves are 

 derived, is at once a result and a proof of the displacement 

 of the branchial cavity. 



How is it that the bivalve Mollusca, of which the Oysters are 

 typical, escaped both the dorsal cone and the coiling which 

 would be its natural sequel ? The method we have just followed 

 will provide the explanation. Being without any embarrassing 

 dorsal cone, those Molluscs, like the Oysters, which do not lie 

 on their sides, are strictly symmetrical, but it is not difficult 

 to discover the nature of their affinities with the other molluscs. 

 The primitive Gasteropods, in fact, present some peculiar 

 structural characters. The heart possesses two auricles, and 

 1 XLIV. 2 XL VI, 201. 



