324 MANUAL OF ZOOLOGY. 



and opening into it, is, in many cases, a blind sac containing 

 a peculiar transparent glassy body, which is known as the 

 " crystalline stylet," but the functions of which are absolutely 

 unknown. The intestine has its first flexure neural, perforates 

 the wall of the heart, and terminates posteriorly in a distinct 

 anus, which is always placed near the respiratory aperture. 

 The liver is large and well developed, but there are no salivary 

 glands. 



There is always a distinct heart, composed either of an 

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

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

 tributed 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 Lamellibranchiata consist 

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

 130, b). In some cases there is only one gill on each side of 

 the body, the external pair of branchiae being absent. 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 cham- 

 ber. 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 " inhalant " and " exhalant " apertures 

 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 (fig. 130, s s), and which can usually 

 be partially or entirely retracted within the shell by means of 

 special muscles, called the "retractor-muscles of the siphons." 

 These siphons are more especially characteristic of those La- 

 mellibranchs which spend their existence buried in the sand, 

 protruding their respiratory tubes in order to obtain water, and 

 with it such nutrient Articles as the water may contain. The 

 presence or absence of retractile siphons can be readily deter- 

 mined merely by inspection of the dead shell. In those 

 bivalves in which siphons are not present, or if present are not 

 retractile, the "pallial line " in the interior of the shell is un- 

 broken in its curvature, and presents no indentation (Integro- 

 pallialia). In those, on the other hand, in which retractile 



