OF CONCHOLOGY. 289 



The study of the embryo of Ancylus enables us with ease to 

 recognize this organ from its origin. In fact, upon the embry- 

 onic sphere the head first betr<ays itself by the formation of the 

 mouth. Soon two disks, bounded by a circular cushion, show 

 themselves, the one near, the other opposite the mouth ; the 

 former is the foot, the latter the mantle. At this moment the 

 Ancylus represents the ideal being with the four principal parts. 

 Starting from this state, we may vary the forms and explain 

 the modifications of the Gasteropod type. But in the first place, 

 to have an exact idea of the mantle, let us suppose the embry- 

 onal disk from which it is derived eminently elastic and exten- 

 sible ; let us assume, further, a traction exserted upon its centre 

 and directed backwards, and we shall obtain a reversed cone, of 

 which the apex will be the point of application of the force of 

 traction, and the base the part of the body bounded by the cir- 

 cular cushion of the primitive disk. The intestines will pene- 

 trate by traction into the cone thus formed ; but the foot and 

 head will remain without. These four parts will be deformed, 

 but their relations will remain constant. 



It is easy to account for some forms which are very different 

 in appearance. For example, in the Limaces the foot increases 

 sufficiently below to lodge the viscera, and the mantle forms 

 nothing more than a little disk or buckler ; in the Testacellse and 

 the Bulbese the foot follows the neck in its excessive develop- 

 ment, and the mantle remains rudimentary at the end of the 

 body ; in the Aplysit^ the foot and the neck become much de- 

 veloped upward, but the foot still increases sufficiently in its 

 lower part to cover with its two lobes the back and even the man- 

 tle, with which it has been erroneously confounded. 



The criterion which I propose enables us to determine the 

 homologous parts. 



Four groups of nervous ganglia characterize the Mollusca in 

 general and the Gasteropoda in particular. These are, first, 

 the stomato-gastric, the cerebroid, and the pedal ganglia. The 

 fourth group, intermediate between the latter two, always placed 

 a little behind and below the pedal centre, is unsymmetrical — 

 that is to say, formed by an uneven number of ganglia, general- 

 ly five. It characterizes the Gasteropod group, and, except the 

 head, the foot, and the viscera, it innervates all the organs. The 

 name which would designate its relation would be that of bran- 

 chia cardio-pallio-genital ; but I shall simply call it the median 

 or inferior centre. It varies much ; sometimes it forms a very 

 small ring, sometimes an extremely long curve which seems to 

 modify and change all other relations. Thus in the Limnseae, 

 the Planorbes, and the Ancyli, although its ganglia are a little 



