180 INVERTEBRATE ANIMALS. 



retractile respiratory siphons, and lived, therefore, most prob- 

 ably imbedded in sand or mud. 



There is always a distinct heart, composed of two or three 

 chambers, and in all cases acting as a mere arterial heart. 

 That is to say, the heart propels the aerated blood derived 

 from the gills through the body, and has nothing to do with 

 the propulsion of the non-aerated or venous blood through the 

 gills. There is never any distinct head in any of the Bivalves, 

 and for this reason they are sometimes called the " headless " 

 (acephalous) Mollusks. The mouth is simply placed at the 

 anterior end of the body, and is never furnished with teeth, 

 though usually provided with membranous processes or 

 "palpi" (Fig. 80, p). The mouth opens into a gullet which 

 conducts to a stomach. The intestine is convoluted, and usu- 

 ally perforates the ventricle of the heart, ultimately terminating 

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

 aperture. A large and well-developed liver is also present. 



The nervous system has its normal form of three principal 

 masses the cerebral, the pedal, and the parieto-splanchnic 

 ganglia. 



The majority of the bivalve Mollusks have the sexes dis- 

 tinct, but they are sometimes united in the same individual. 

 The young are hatched before they leave the parent, and, 

 when first liberated, are ciliated and free-swimming. 



The habits of the Lamellibranchiata are very various. 

 Some, such as the Scallops (Pecten), habitually lie on one 

 side, the lower valve being the deepest, and the foot rudimen- 

 tary or wanting. Others are fixed to the bottom of the sea 

 by "the substance of one of the valves. Others, such as the 

 common Mussel, are moored to some foreign object by a tuft 

 of silky fibres, constituting a " byssus." Many, such as the 

 Gapers (My a] and Razor-shells (Solen), spend their existence 

 sunk in the sand of the sea-shore or the mud of estuaries. 

 Others, such as the Pholades, bore holes in rock or wood, in 

 which they live. Finally, many are permanently free and 

 locomotive. 



CLASS II. GASTEROPODA (Gr. gaster, belly ; podes, feet). 

 This class includes an enormous number of Mollusks, such as the 

 land-snails, sea-snails, whelks, limpets, slugs, sea-lemons, etc., 

 which agree in many fundamental characters, but nevertheless 

 present many striking differences. From the very common 

 occurrence of a shell composed of a single piece, the Gastero- 

 poda are often spoken of in a general way as the " univalve " 



