8 
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 
Sijpunculus Johnstoni and swarms of Leucodore and Nicomache 
occur, while Nereis cultrifera , Eulalia , Syllis , 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 in 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 
Saxicavce 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. 
If 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 
Membraniporce and the amethystine streaks of Helcion jpellu- 
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 algae 
cluster on the stalks of the seaweeds. Hosts of coal-fishes 
swim through and among them ; and entangled Medusae , now 
at the mercy of the tide, are lifted on the rock and again 
washed off. Scarlet Solasters and more soberly coloured 
Urasters 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 
