*i INTRODUCTION. 



in the muddy or sandy crevices a safe retreat for their soft 

 bodies, slender tubes, or muddy tunnels, and opportunities 

 for capturing sufficient food at the free margin of the. rock or 

 from the ingoing currents. It is chiefly in such localities that 

 SijiNHi-itliis Johnstoni and swarms of Leucodore and Nicomache 

 occur, while Nereis cultrifera, Eulalia, Sylh's, and the nemer- 

 teans are also common. Occasionally an Idotea is met with ; 

 but by the general absence of the isopods these crevices are 

 distinguished from those in the gneiss of the south and west, 

 as iu the Channel Islands and the Outer Hebrides ; and they 

 are especially distinguished from the former by the absence 

 of Pilumnus hirtellus, Area, the Sabellidce, the Eunicidce and 

 their allies. To these fissures certain boring annelids and 

 SaxicavcB chiefly retreat when the rocks do not afford a 

 .suitable medium for their perforations — though at St. Andrews 

 there is free scope in this respect, from the sandstones and 

 shales so soft as to be pitted deeply by the common limpet to 

 those nearly as dense as granite. 



[f further, under the favourable ebb of a spring tide, the 

 water has receded to an unusual degree, the observer may look 

 down from the jutting rocks on the rich laminarian forests that 

 flourish in a region of perpetual flood, and watch the silvery 

 MembraniporcB and the amethystine streaks of llelcion ■peJlu- 

 cidum on their flattened bands, which wave and curl with 

 every surge of the sea ; and just as the trunks of the forest- 

 trees are incrusted with lichens, so feathery tufts of red alga 

 cluster on the stalks of the seaweeds. Hosts of coal-fishes 

 swim through and among them ; and entangled Medusa, now 

 at the mercy of the tide, are lifted on the rock and again 

 washed off. Scarlet Solasters and more soberly coloured 

 Urastcrs contrast strongly with the dark water and olive-green 

 seaweeds ; and nothing can be more beautiful than the light 

 purplish pink of the sea-urchins as they progress by aid of 

 their sucker-feet along rock, stone, or laminarian root. 



Moreover, in the deeper -water off* the bay the fishermen 

 occasionally secure rare fishes, entangle a shark, a bonito, a 

 porpoise, a seal, or even a great northern diver in their nets, 

 and bring many rare invertebrates to the harbour in their boats. 

 The crab-pots, salmon-nets, and trawls further add their quota 



