CHAPTEE XIII. 



THE SEA-SLUGS. 



General characters of Opisthobranchs The sea-hare The sea-lemons, 

 or Dorids Five common species The spawn and breeding habits 

 Development Goniodoris, its structure and habits Some other 

 sea-slugs General characters of the colouring Their inedibility 

 and its causes The Eolids Three common species General notes 



WE now come to a singularly interesting and beautiful 

 group of Gasteropods, mostly without shells, and often 

 of very singular shape. They constitute the group of the 

 Opisthobranchs, and, as already seen, are characterised by 

 the fact that the heart is in front of the gill when this 

 is present, instead of being behind it, as in the Gasteropods 

 just considered. The greater number of these shell-less Gas- 

 teropods are often called sea-slugs, or Nudibranchs ("gills 

 exposed"), and certain sea-slugs are abundant on every 

 shore. Most of them, especially the smaller kinds, live 

 well in confinement, and should be studied in the living 

 condition. They do not preserve well, both colour and 

 shape being usually lost even under favourable conditions, 

 and they are rarely to be found in museums ; so that unless 

 you draw and describe your specimens as you find them, 

 there is little chance that you will be able to name them 

 afterwards. Again, many of them seem to be more or less 

 migratory in their habits, and are not found between tide- 

 marks except at the breeding season. As this usually falls 

 in the colder months, you can hardly hope to find such 

 species if your visits to the shore are confined to the 

 summer. In March, for example, I have seen the shore 

 rocks whitened by the spawn of forms which in summer 

 are rare, but at this time occurred in clusters of five or six 



248 



