472 PHILOSOPHY OF ZOOLOGY. 



the border, is a rov of conical tubercles. The true tenta- 

 cula are placed- towards the base of this fin near the neck. 

 Each of them consists of a small fleshy cone, striated across, 

 with a semicircular sheath behind. The branchiae consist 

 of a tapering, fleshy stalk, spirally twisted towards the sum- 

 mit with a series of filaments on one side. They are four- 

 teen in number on each side, alternately and oppositely 

 small and large. The anus opens in front of the third 

 branchla on the right side. The orifice of generation is 

 exhibited under the first branchia of the same side. In 

 front of each of the larger branchiae, is a small cavity with 

 a small filament in the centre. 



The mouth is situate underneath the tentacula. It 

 consists of a large funnel, covered within with soft papillae, 

 destitute of jaws or tongue. The gullet is short, the 

 stomach simple, fleshy, and covered with a thick cuticle. 

 The salivary glands are slender and branched, and open 

 into the gullet. The intestine is likewise short, and pro- 

 ceeds directly to the anus. The liver pours the bile into 

 the canal at the pylorus ; and likewise sends out another 

 duct, which opens externally near the anus. The organs 

 of generation are similar to the doris. 



The T. fimbria is the type of the genus, a figure of 

 Avhich, with its anatomical details, it given by M. Cuviek, 

 in his Memoire siir le Genre Thethys. 



2. Body concealed in a Spiral Shell. 



40. This section includes the genus Valvata of Mul- 

 LER, represented by two British species V. cristata {Helix 

 cristata of Montagu), and V. piscinalis {Turbo Jbntina- 

 lis). These resemble in aspect the aquatic pulmoniferous 

 gasteropoda. The branchiae appear in the form of a fea- 

 ther, with a central stem, and a row of compound branches 

 on each side, decreasing in size from the base to the free 

 extremity. It issues from the neck near the middle, a 



