MOLLUSCA: LAMELLIBRANCHIATA. 253 



for the animal to relax tlie muscles which are provided for the 

 closure of the valves, whereupon the elastic force of the liga- 

 ment and cartilage is sufficient of itself to open the shell. 



The body in the Lamellibranchiata is always enclosed in an 

 expansion of the dorsal integument, which constitutes the 

 * mantle' or 'pallium,' whereby the shell is secreted. The 

 lobes of the mantle are right and left, and not anterior and 

 posterior as are the mantle-lobes of the Bracbiopoda. Towards 

 its circumference the mantle is more or less completely united 

 to the shell, leaving in its interior, when the soft parts are re- 

 moved, a more or less distinctly impressed line, which is called 

 the ' pallial line,' or 'impression.' 



There is no distinctly differentiated head in any of the Lo- 

 mellibranchiata, and the mouth is simply placed at the ante- 

 rior extremity of the body. It is furnished with membranous 

 processes or ' palpi ' (usually four in number), but there is no 

 dental apparatus. The mouth opens into a gullet, which con- 

 ducts to a distinct stomach. The intestine has its first flexure 

 neural, perforates the wall of the heart, and terminates pos- 

 teriorly in a distinct anus, which is always placed near the 

 respiratory aperture. The liver is large and well developed. 



There is always a distinct heart, composed either of an au- 

 ricle and ventricle, or of two auricles and a ventricle. The 

 ventricle propels the blood into the arteries, by which it is 

 distributed through the body. From the arteries it passes 

 into the veins, and is conducted to the gills, where it is aerated, 

 and is finally returned to the auricles. 



The respiratory organs in all the Lamellibrancliiata consist 

 of two lamelliform gills, placed on each side of the body. The 

 gills are in the form of membranous plates, composed usually 

 of tubular rods, which support a network of capillary vessels, 

 and are covered with vibrating cilia, whereby a circulation of 

 the water is maintained over their surfaces. In some bivalves 

 the margins of the mantle are united to one another, so that a 

 closed branchial chamber is produced, and in the others the 

 arrangements for the admission of fresh, and the expulsion of 

 effete, water are equally perfect, though there is no such 

 chamber. In those in which the mantle-lobes are united at 

 their margins, there are two orifices, one of which serves to 

 admit fresh water, whilst the effete water is expelled by the 

 other. The margins of these * inhalent ' and ' exhalent ' aper- 

 tures are often drawn out and extended into long muscular 

 tubes or ' siphons,' which may be either free, or may be united 

 to one another along one side, and which can usually be, par- 

 tially or entirely, retracted within the shell by means of special 

 muscles, called the ' retractor-muscles of the siphons.' These 



