THE SEA SHORE 33 



which may be several inches thick and vary in colour from 

 a pure white in shaded places to a yellow or green in more 

 exposed situations. The deep crimson Hymeniacidon is 

 often found with it. A common sponge which is attached 

 by a stalk is the purse sponge [Grantia compressa), often 

 found on the sides of rocks where there is no danger of 

 it being dried up, and there are other kinds of sponges too 

 numerous to describe. Growing on rocks or fucoid fronds 

 is the tiny worm Spirorbis, which lives in a flat spiral shell 

 like that of a snail and often occurs in great numbers. 

 Larger worms with limy tubes of irregular shape (Serpulids) 

 are frequently common on rocks. Frequently covering 

 rocks are found sea squirts, especially the compound forms — 

 consisting of colonies of simple individuals — such as the 

 golden-stars Tunicate {Botryllus), which closely resembles 

 a sheet of purple jelly dotted with tiny golden stars (Plate 12) 

 though some simple sea squirts are found, especially the 

 Gooseberry {StyUopsis grossularia) , a little reddish lump 

 well described by its oommon name, and abundant near 

 low-water mark (Plate 12). Other encrusting animals are 

 the sea mats or polyzoa. which form, with the sponges, sea 

 squirts, coralline weeds and other algae, a dense carpet 

 over the face of the rocks. Finally about low-water mark 

 we may find bivalve molluscs such as the saddle-oyster 

 [Anomia), the common mussel {Mytilus edulis) and one of 

 the smaller scallops such as Pecten varius. These are 

 attached in ditferent ways, the saddle-oyster being per- 

 manently attached by its shell and adhering very closely to 

 the surface of the stone while the mussel and scallop are 

 secured by tough threads known as " byssus " the formation 

 of which is described on page 304. 



A fresh fauna is revealed when we begin to turn over loose 

 stones. Near high-water mark there is a varied collection 

 of small crustaceans and insects. Of the former the largest 



