STRUCTURAL MODIFICATIONS 139 



its ventricle is traversed by the rectum, and they have two 

 bipectinate gills. These special characters are found again in 

 the Lamellibranchs, which ought, in consequence, to be 

 regarded as related to the older Gasteropods called Diotocardiac, 

 because of the two auricles with which their heart is provided. 

 As a matter of fact, all the present Diotocardiacs are dis- 

 symmetrical ; the most primitive of them all, the Pleuroto- 

 mariidce, the Fissurellidae, Haliotidae, and a few others have 

 retained as a common character the most manifest traces of 

 bilateral symmetry ; the anterior edge of the opening of their 

 shell is either deeply divided or curved inwards, at any rate, in 

 the young ; the slit may persist (species of Pleurotomaria, 

 Emarginula) and give rise to a series of holes arranged in a 

 helicoid line (Haliotis), or to an opening situated at the apex 

 of the shell, which is then in the form of an elliptical cone 

 (Fissurella). However, this slit, which indicates a division of 

 the mantle into two lobes, each connected with the two gills, 

 is also found, arranged according to the symmetrical plane of 

 the shell, in Bellerophon of the earliest Primary Period, now 

 extinct. There can be no doubt that these organisms were 

 diotocardiac Gasteropods, which, like the Cephalopods, all 

 swimmers, had preserved a perfect bilateral symmetry. But 

 for our purpose it is enough that organisms analogous to 

 Bellerophon, in the course of their pelagic life, gradually 

 re-absorbed their dorsal cone, and that the initial slit became 

 extended along the whole length of the plane of symmetry 

 in order that the shell should become bivalve. The play of the 

 muscles in closing the two valves of the shell compressed the 

 mollusc, which, having lost its dorsal cone, was able to crawl, 

 like the Solenmyidae, without any alteration of its bilateral 

 symmetry. The disappearance of the dorsal cone, moreover, 

 is a frequent phenomenon among the crawling Gasteropods, 

 and occurs in the most varied orders of this group ; it is, for 

 instance, complete in the Fissullidae, which are diotocardiac, 

 the Patellidae, which are heterocardiac, the Valvatidae, which 

 are monocardiac, the Limacidae and the Vaginulidae, which are 

 pulmonate, and among a large number of Opisthobranchs. 

 To suppose that it had disappeared among Molluscs analogous 

 to Bellerophon is therefore only to base our hypothesis on quite 

 a common phenomenon. 



Thus the three main classes of Molluscs are easily explained. 



