GASTROPODS. 



359 



mucus, and the fry generally emerges in about ten days or a fortnight. Like 

 Scyllcea, the animals known as Glaucus live in the open sea upon floating 

 seaweed, and feed upon jelly-fish of various kinds. The form of the body 

 is peculiar, having three lobes or expansions on each side, supporting the linear 

 branchiae. Pelagic animals generally seem to be either colourless or specially 

 coloured, with a view to protection from enemies both above and below the surface 



jEolidia papillosa. 



of the water. Probably the blue colour of lanthina is protective, as resembling 

 that of the ocean water. The blue tint of Glaucus may also serve this purpose. 



This group of Nudibranchs comprises a number of forms which 

 have no external branchiae, but effect respiration through the general 

 ciliated surface of the body. A typical example of this section is illustrated by 

 the accompanying figure of Elysia viridis which is a little creature about an inch 



Pellibranchiata. 



Elysia viridis (magnified). 



in length, of a dark green colour, varied with white, greenish blue, and reddish 

 white dots. It lives upon seaweed, and, owing to its colour, is difficult of obser- 

 vation. The absence of external branchiae is noticeable, and the ramified blood- 

 vessels seen in the lateral expansions of the body are presumed to be connected 

 with the respiratory function. This is the only species found upon the shores of 

 Britain, and is most common on the south and south-west coasts. It is said to mimic 

 the colour of the different seaweeds upon which it occurs. Limapontia is another 



