40 



THE SEAS 



is frequently mixed, burrowing forms being by far the 

 commonest. Thus, of the bivalves, the large " gaper," 

 Mya truncaia, is the most conspicuous, it possesses a thick 

 sh'ill — some three times the size of the mussel — which has 

 a permanent gape at the hind end where the long, muscular 

 siphons project (Fig. 4). Another 

 characteristic type is Scrobicularia, to 

 be recognized by its extraordinarily 

 flattened shell and by the length of the 

 siphons which, unlike those of the gaper, 

 are free from one another. Where 

 the mud is intermingled with stones 

 the large horse-mussel {Modiolus) is 

 common. Worms are abundant, es- 

 pecially burrowing species, and those 

 living in tubes ; of these the common 

 Sabella pavonina, with its rubber- 

 like tube and widely spiead ring of 

 red and white plumy tentacles, and 

 Myxicola infundibulum, wdth its thick 

 gelatinous tube and shorter, more com- 

 pact tentacles, are the commonest. 

 The former often occurs in such num- 

 bers that at low water the tubes, of 

 which usually some five inches project 

 above the surface, appear like a little 

 forest. Of anemones, the brown 

 Sagartia bellis is the commonest. 

 There are a variety of crustaceans 

 including the ubiquitous scavenging shore crab. Estuaries 

 and creeks with a mixed bottom of mud and stones often 

 harbour oysters, which are fastened to the stones, and wath 

 these are found their invariable enemies, the boring dog- 

 whelks, Nassa and Murex and the common starfish, Asterias 



Fig. 4.— The Gaper 



(Mya trimcata). 



