262 LIFE BY THE SEASHORE. 



The sea-slugs are in many ways a most interesting group, 

 and well worth careful attention. First as to structure. 

 With the exception of Aplysia, all those we have named are 

 without trace of shell; but this is not universally true of 

 Opisthobranchs, for some of them have well-developed 

 shells. The shell has indeed been gradually lost, as in so 

 many groups of Mollusca. Then, again, the sea-hare has a 

 typical gill like that of whelk or periwinkle ; Doris has a 

 circlet of many gill-plumes, and these gradually decrease in 

 number as in Polycera and Ancula, till we come to forms 

 with no gills at all. Simultaneously with the disappearance 

 of the gills we have the appearance and increase of the 

 curious papillae, branched as in Dendronotus, or simple as in 

 Eolis, which help to give the Opisthobranchs their quaint 

 and beautiful shapes. Similarly, we see in passing from 

 Aplysia towards the Eolids how the solidity of appearance 

 which we are accustomed to associate with our shore 

 Gasteropods gives way to a delicate translucency or trans- 

 parency, and the dull tints of whelk or periwinkle to soft, 

 bright colours, which are sometimes like those of the 

 surroundings, and sometimes markedly different from these. 

 Generally, we may say that the Opisthobranchs are a 

 specialised group of Gasteropods, which in some cases have 

 lost many of the Gasteropod characters, but which can be 

 shown to have originated from typical forms with coiled 

 shell, visceral hump, gill, and characteristic asymmetry. 



