230 LIFE BY THE SEASHOKE. 



markings are clearly seen. Each has a central ridge orna- 

 mented by coarse longitudinal lines and ending in a beak, 

 and two lateral areas ornamented by curious " tear-shaped " 

 granules, whose pointed ends are directed towards the 

 beak. The sculpture as a whole is interesting and very 

 characteristic. 



Another species, C. ruler, easily recognised but not very 

 common, is of a bright, shining red colour, marked and 

 variegated with white. The surface of the valves is per- 

 fectly smooth and without trace of sculpture. 



By far the commonest species is C. marginatus, which is 

 abundant everywhere on shore rocks. It is almost confined 

 to the littoral zone, and is very variable. 



After the Chitons we come to the Gasteropods proper, in 

 which the shell, when present, is always simple and often 

 coiled. The classification is a matter of some difficulty, for 

 those now in use depend upon anatomical details which are 

 somewhat beyond our scope. We shall consider the true 

 Gasteropods as divided into three orders: (1) the Zygo- 

 branchia, (2) the Azygobranchia, (3) the Opisthobranchia. 

 The first order includes the limpets, of which there are 

 many kinds. Sometimes two gills are present, sometimes 

 only one, as in Acmc&a, while in yet other cases, as in 

 Patella, there is no gill at all. The name, which signifies 

 " gills paired," is therefore a little deceptive. The shell is 

 usually cap-shaped, and never more than very slightly 

 coiled. In the general case there is little difficulty in 

 recognising the common limpets. 



The Azygobranchia ("gills unpaired") include the great 

 majority of the snail-like Gasteropods of the shore. The 

 shell is usually large and coiled, and there is a single gill. 

 The third order, the Opisthobranchia ("gills posterior"), 

 includes forms which are often not easy to recognise as 

 Molluscs at all. The shell is often absent, and is never well 

 developed. When a gill is present it is placed behind the 

 heart, instead of in front of it as in other Gasteropods ; but 

 usually there is no true gill, its place being taken by out- 

 growths of the mantle. Often brilliant in colour and quaint 

 or beautiful in form, there is at least no fear of confusing 

 the Opisthobranchs with other Gasteropods. They are 

 very abundant on the shore, especially at certain seasons 



