MARINE MOLLUSCS 



235 



water molluscs. "When the animals are dead it will be found that 

 their gills are more or less extended, sometimes fully so, and they 

 may then be transferred to diluted spirit or a two per cent, solution 

 of formaldehyde. 



In fig. 162 we represent four species. Two of these Triopa 

 claviger and Mgirus punctilucens belong to the family Doridce, 

 the members of which are popularly known as Sea Lemons, and 

 are distinguished by the presence of plume-like gills situated on the 



FIG. 162. NUDIBBANCHS 



1. De ndronotus arborescent. 2. Trifonia plebeia. 3. Triopa claviger. 

 4. dSgirus punctilucens 



middle of the back. Another family (Tritoniadce), characterised 

 by the arrangement of the gills along the sides of the back, and by 

 tentacles that can be retracted into sheaths, is represented by 

 Tritonia plebeia and Dendronotus arborescens in the same figure, 

 and by Doio ooronata in fig. 161. The family Molidce also 

 have their gills arranged along the sides of the back, but they differ 

 from the last in that their tentacles are not retractile. They in- 

 clude the two species numbered 3 and 4 on fig. 161. The remaining 

 one on fig. 161 Elysia viridisis a member of the family PhilU- 



