432 THE ANATOMY OF IXYERTEBRATED ANIMALS. 



number of long tentacles. These protrude through the an- 

 terior opening of the mantle, and play the part of prehensile 

 organs. Behind and below the oral cup the very long sub- 

 cylindrical foot proceeds. Near its extremity are two lateral 

 fleshy lobes which perhaps correspond with the epipodia of 

 other Mollusks. The oral cup leads into a buccal chamber 

 containing the odontophore, whence the oesophagus passes 

 to the stomach. The liyer consists of two symmetrically- 

 branched divisions ; and the intestine, after becoming coiled 

 upon itself, ends in a prominent anal papilla, in the median 

 line, behind the root of the foot. There is no heart, but the 

 blood fills spacious sinuses. There are no special respiratory 

 organs distinct from the wall of the pallial cayity. The two 

 renal organs open one on each side of the anus. The renal 

 blood sinus communicates directly with the pallial cayity by 

 two apertures, situated close to those of the renal organs. 

 In the neryous system, the commissures of the parieto- 

 splanchnic ganglia pass directly to the cerebral ganglia, as 

 in the Lamellibranchs. The sexes are distinct, and the geni- 

 tal gland is single and symmetrical, though its duct opens 

 into the right renal organ. The embryo is at first surrounded 

 by a number of ciliated rings, its anterior end presenting a 

 tuft of long cilia. By degrees the cilia become restricted to 

 the edges of a disk, into \yhich the anterior end of the embryo 

 expands, and which represents the pr[^-oral ciliated yelum of 

 the Lamellibranchs. The mantle now appears on the dorsal 

 aspect of the bod}^, behind this disk. Its yentral edges are 

 free, and it secretes a shelly plate of corresponding form. 

 But, as deyelopment advances, the edges of both mantle and 

 shell unite in the median ventral line, leaving the anterior and 

 the posterior ends open. 



The Scapliopoda are an ancient group, remains of them 

 occurring as far back as the Devonian epoch. 



The higher Odontophora (or the Gasteropoda^ Pteropoda^ 

 and Cephalopoda of Cuvier) fall into two divisions, according 

 to the structure and arrangement of the parts of the foot. 

 In the one division (the Gasteropoda and Pteropoda^ it may 

 be a simple disk, or it may be divided into three portions — 

 an anterior (the propodiurti)^ a middle (the mesopodhi7)i), 

 and a posterior (the meta2)odhan) ; and it may be still further 

 complicated by the development from its sides of muscular 

 expansions — the epipodia. But, whatever the shape of the 

 foot in these Mollusks, its margins are not produced into 



