200 



INVERTEBRATE AXIMALS. 



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) Molluscs. 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. 1.39, Ip). The mouth opens into a gullet 



F.g. 139.— L;;inel]ibiain-hiata. Diagrammatic representation of the anatomy of a s'phon- 

 ate Bivalve. The left vahe and left )nantle-lobe are removed, and the siphons 

 are cut short. il Umbo; ca Cartllagc-pit ; o Month; Ij) Labial palpi; et 

 Stomach, surrounded by liver ; st Sac containing the crystalline stylet ; i i In- 

 testine, perforating the heart (/i) ; r Rectum, terminating in the anus ; ad An- 

 terior adductor ; pd Posterior adductor ; n Suprau^sophageal or cerebral ganglion 

 (the mouth is a little displaced upwards, so that the ganglion comes to lie below 

 the gullet insf(;ad of above it): n' Parieto-splanchnic or branchial ganglion; 

 / Font; XX Cut ed;_;e of the right mantle-lobe; rs Retractor muscle of the 

 siphons; br Brancbite. of the left side; g Renal organ ("Organ of Bojanus"); 

 s Inhalant siphon ; s' Exhalant siphon. 



which conducts to ii stomach. The intestine is convoluted, and 

 usually perforates the ventricle of the heart, ultimately terminating 

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

 ture (fig. 1.39, r). A large and well-developed liver is also present. 



The nei'vous system has its normal form of three principal masses 

 — the cerebi'al, the pedal, and the parieto-splanchnic ganglia. 



The majority of the Bivalve Molluscs have the sexes distinct, but 

 they are sometimes united in the same individual. The young are 



