THE NERITIC ZONE- LIFE IN SHALLOW WATER 439 



protected against the buffets of the waves when withdrawn into 

 their thick white dwellings; and the Periwinkles {Littorina), 

 including the edible species (Z. littorea) ; a smaller, more rounded 

 kind (Z. obtusata), often of bright -orange hue, which crawls 

 over brown sea-weeds; and a third sort (Z. rudis), that dwells 

 near high-water mark and has its breathing organs modified in 

 consequence (see vol. ii, p. 459). Sea- Lemons {Doris) and other 

 marine slugs are also common. 



Among neridc Bivalves {Laj7iellibranchid) forms of economic 

 importance may be mendoned, such as the Oysters {Ostrea), 

 attached by the substance of one valve; the Edible Mussels 

 {Mytiliis), moored by silky byssus threads ; the Scallops {Pecten), 

 some of which can swim by opening and closing their shells; and 

 the Cockles {Cardium), which burrow in the sand. Other delvers 

 in sand or mud are the Gapers {Mya), the Razor-Shells {Solen), 

 and many more; while Piddocks \Pholas) and Date-Shells {Litho- 

 domus) are able to excavate dwellings in stone. 



Of Primitive Molluscs {Amphineurd) the flattened Mail-Shells 

 {Chiton) live under stones or in rock-crevices. 



Neritic Crtutaceans {Crustacea). — Prominent among these 

 are the Prawns, Shrimps, Lobsters, and Crabs, of many species. 

 Some forms of the last kind which we commonly see on our 

 own coasts are the Edible Crab {Cancer paguj'us), the green 

 Shore-Crab {Carcinus incenas), and, near low-tide mark, the little 

 flattened Porcelain Crabs {Porcellana). 



Neritic Annelids {Annelida). — Of these there is a vast host. 

 Of British forms may be mentioned the actively-creeping Sea- 

 Centipedes {Nereis) and many related species; the Sea-Mice 

 {Aphrodite), short plump worms with beautiful iridescent bristles; 

 Scale- Worms {Polynoe) ; Lug- Worms {Arenicold), that burrow in 

 sand or mud; Sand-Worms {Sabellaria), living in communities 

 and gluing grains of sand into dwellings; and various species 

 sheltered in white calcareous tubes, sometimes irregular in shape 

 {Serpula), or coiled into small flat spirals {Spirorbis) attached to 

 brown sea-weeds. 



Other Worm-like Animals of the Neritic Zone. — Here may 

 be mentioned, in passing, the colonial Moss-Polypes {Polyzoa), 

 of which the branching skeletons are often taken for sea-weeds; 

 Nemertine Worms {Nemertea), slimy unsegmented creatures often 

 found coiled up under stones; Siphon-Worms {Sipuncnlus), that 



