306 THE BRITISH NATURE BOOK 



glistening shells, some smelling distinctly of garlic. This particular species is 

 white. 



The genus Pupa contains species with minute brown shells, like seeds. 



At this point we may as well deal with a few curious members of the 

 Gasteropoda, which for want of a better name we may well call " Sea Slugs." 



They resemble to some degree our land slugs, but they are not by any means 

 closely related ; and as they carry their breathing apparatus exposed on the 

 back and sides, they are called Nudi-branchiata, or Naked-gilled molluscs. 



They may be looked for especially on rocky coasts, and if placed in clear 

 water the beauty of their tentacles and the branchial plumes which form their 

 lungs will be seen. 



A common example is the Sea Hare (Aplysia depilans), which when con- 

 tracted appears a rounded mass of brown jelly, but when expanded has a head 

 and tentacles like the head and ears of a hare. It is some 3 inches long, and 

 possesses the power of exuding a purple fluid, once considered to be deadly 

 poison. It is, however, entirely harmless. 



The Sea Lemon (Doris tuberculata) is about the same length, resembling 

 the half of a cut lemon, both in its colour and warty surface. It has eight 

 gills or branchiae, feathery, and like the petals of a flower when expanded. 

 There are several hundred species, of which Doris johnstoni, the Sponge Slug, is 

 creamy in colour, spotted brown, like a sponge in appearance and touch, and 

 feeds on sponges. 



The Crowned Eolis (Eolis coronata), found under stones, is the most beautiful, 

 having rows of scarlet and blue gills crossing the back. It devours sea anem- 

 ones and zoophytes. 



Fresh-water Molluscs. 



It will be more convenient to group together the various shells to be found 

 in fresh water. Both in rivers and ponds certain molluscs are to be found 

 which are closely allied to the more familiar marine species. Four species of 

 fresh-water mussel belong to the British Isles, three belonging to the genus 

 Unio, the fourth to Anodonta. The Unios have the two valves of the shell 

 united by strongly marked hinge teeth, but Anodonta, as its name implies 

 (Greek an, " without ; " odontes, " teeth "), has none. Another characteristic 

 feature which helps to distinguish the two classes lies in the fact that all Unios 

 have thick shells, while Anodonta possess very thin ones. 



The Swan Mussel (Anodonta cygnea) is 4 to 6 inches long, greenish brown 

 in colour, though often with many white patches, where the surface layer has 

 been removed. There are plainly marked lines running parallel with the 

 margin, and these, be it again noted, are the " lines of growth." 



The Painter's Mussel (Unio pictorum) is so called because once its shells 

 were used for holding water-colour paints. 



The Pearl Mussel (Unio margaritifer) is the fresh-water mussel which pro- 

 duced the pearls (and still does so in Scotland, Wales, and Ireland), for which, 



