130 OYSTERS, AND ALL ABOUT THEM. 



higher orders in having no heads, and are called acephalous. 

 They have mouths, and a nerve-knot above this ; but the 

 mouth is not prominent, and lies far within the shelly box, 

 and often between the soft projections of the body, which 

 extend some distance beyond it. Their organs of sense 

 are also very poor and imperfect ; and when they are 

 possessed at all, they are placed in other parts of the body. 

 There are two classes, viz : Brachiopoda and Conchifera, 

 which together compose this group of headless, two- 

 shelled Mollusca, and which are extremely different in 

 structure throughout. There is, however, an external 

 difference by which they may be distinguished by a super- 

 ficial observation. 



In the Brachiopods the shells are secreted on 

 extensions of the membrane of the body on the back and 

 front of the animals ; while in the Conchifera the shells 

 are placed upon membranes which are developed from the 

 sides of the animal. Thus, if both animals were split down 

 the middle by a cutting instrument, which should leave the 

 divided halves more or less alike, that instrument would 

 pass through both shells of the Brachiopods, dividing them 

 both into two equal parts, while it would not cut the shells 

 of the Conchifera at all, but only sever them the one from 

 the other. So that in this arrangement of the animal to 

 the shell, the separating process would leave two absolutely 

 similar halves. On the other hand, the division of the 

 Brachiopod, so as to sever the shells, would leave two 

 portions unlike both in size and form. A similar partition 

 of the conchiferous shell would leave two equal and similar 

 parts. 



It should, however, be remarked that the ordinary 

 position occupied by both classes of bivalves, resting as 

 they do on the bottom of the sea, sometimes interferes 



