MANUAL OF NATURAL HISTORY. 139 



II. S UB- CLASS. — Opisthobranchiate-Gasteropods 

 (Opisthobranchiata). 



Heart placed before gills ; gills plume-like, on the 

 side under the mantle, or arranged along the back, 

 exposed. 



I. ORDER. — Tectibranchs (Tectibranchiata). 



Gill forming a plume on the side under a fold of 

 mantle, usually protected by a shell. 



1. Family. — Umbrella -Shells (Pleurobranchidse). 



Tentacles ear-shaped ; eyes sessile on their 

 inner bases ; gill on the side, under mantle ; 

 shell external or internal, membranous or 

 calcareous. 



2. Family. — Sea -Hares (Aplysiidse). Tentacles 



separate, ear-like ; eyes sessile on head ; foot 

 with large lateral lobes, usually folded across 

 back ; shell internal. 



3. Family. — Long -tailed Bullas (Lophocercidse). 



Tentacles distinct, ear-like ; eyes sessile ; 

 organs of generation close together in one 

 tubercle ; shell external, partly covered by 

 lobes of foot. 



4. Family. — Bullceas (Philinidae). Tentacles form- 



ing a square frontal disk with lobes ; eyes 

 none ; shell concealed in substance of mantle. 



5. Family.— Bubble -Shells (Bullid^). A tentacu- 



lar frontal disk, notched behind ; eyes sessile 

 on the middle ; foot -lobes covering sides of 

 shell : mouth of shell entire in front. 



