4C8 PHILOSOPHY OF ZOOLOGY. 



organ, but so interwoven therewith as to ekide the obser- 

 vation of the anatomist. 



It is obvious, from the structure of the digestive organs, 

 that the species subsist on soft food, requiring neither cut- 

 ting nor grinding, and, in this respect, differ remarkably 

 from the species of the genus Tritonia, which were former- 

 ly arranged along with them. 



The organs of generation differ little from the other her- 

 maphrodite gasteropoda. The vesicle furnishes two canals, 

 one of which goes to the testicle, the other to the penis. 

 There is likewise a minute bag connected with the canal 

 of the latter. The spawn is deposited on sea-weeds and 

 stones. It is gelatinous, of a white colour, and in appear- 

 ance resembles the Spongia compressa. 



The following species are natives of the British seas : 

 1 . D. Arffo ; 2. Verrucosa ; 3. Lavis ; 4. Marglnata ; 

 5. Nodosa ; 6. Qiiadricornis ; 7. Nigricans. 



52. PoLYCEUA. Oral tentacula more than two. 



The branchiae, when withdrawn, are protected by two 

 scales. The superior tentacula resemble those of the doris, 

 the oral ones are more numerous, sometimes amounting to 

 six. P.Jlava and pennigera are British examples 



b. Anus situate on the right side, and unconnected with 

 the branchiae. 



(A.) Mouth furnished with corneous jaws. 



These jaws are in the form of narrow plates, which cut 

 the food by crossing each other hke the blades of a pair of 

 scissors. 



(1.) Branchiae disposed along the back or sides, and un- 

 connected with membranaceous expansions. 



(a.) Tentacula limited to two in number. 



53. Tergipes. Branchiae furnished with a sheath at the 

 base. 



The branchiae form a sinele row on each side, and are 



