SPECIAL PROBLEMS OF SHORE LIFE 67 



whole surface is exposed to wave action and the rocks are 

 barren, but where the dip is high and seaward there may be 

 a good fauna on the more sheltered landward slope. By the 

 adoption of a home in the sheltered niches on the landward 

 side of these rocks the risk of dislodgment is greatly reduced. 

 Such situations are largely chosen for the act of spawning, 

 as will be seen in a later chapter. Interesting are the ways 

 in which the labours of certain forms are instrumental in 

 providing shelter for others. For instance, the tunnels 

 bored by Saxicava rugosa in limestone rocks are occupied 

 by the Polychast Eulalia viridis which is very numerous 

 where such shelter occurs. Again, the activity of the 

 common tube-worm Sahellaria alveolata results in the 

 formation of elaborate grottos which afford protection to a 

 whole association of shore forms (see Plate V). 



2. Special Powers of Adhesion. — Another marked charac- 

 teristic of shore animals is the capacity to adhere closely to 

 the substratum. Some, e.g. sponges, zoophytes, Tunicates, 

 remain permanently fixed, and once detached are incapable 

 of regaining foothold and like uprooted plants are doomed 

 to perish. Others, e.g. anemones, though normally fixed, 

 are capable of securing fresh hold if chance so allows, and, 

 in certain cases, are even capable of a limited amount of 

 locomotion. 



Others again, viz. Gasteropods, during their progress 

 cling tightly to the rocks or weeds by means of the broad 

 foot. 



The regions of attachment of sedentary animals are 

 liberally supplied with mucus glands, but, according to 

 Davis and Fleure (1903), it is doubtful whether the secretion 

 of adhesive material is always of first-rate importance. In 

 the anemones adhesion is secured by the exceedingly close 

 apposition of the pedal disc to the substratum, the tissue 

 following every rugosity of the underlying surface. The 

 limpets probably adhere in the same way, but according to 

 the writers just mentioned the method of adhesion is not 

 known with certainty. They say that " limpets will hold 

 on very tenaciously to a surface smaller than the foot, so 



