CHAPTER VIII 



The Movements of Shore Animals 



The animals of the shore may be divided into two great 

 physiological groups : the sessile and the freely moving. 

 Between the permanently sessile forms such as sponges, and 

 the essentially roving forms such as Crustacea there is every 

 grade of activity. Sessile forms are relatively very numerous 

 on the shore ; indeed, so widespread is this sedentary habit 

 among invertebrates generally that as Willey (191 1) suggests 

 " something peculiarly primordial " would seem to lie at 

 the back of it. This is an interesting point, but we shall 

 reserve further discussion of it until we come to deal with 

 behaviour in the wide sense. The advantages of the 

 sedentary habit have already been seen. We may note, 

 however, that there is no such thing as a sedentary terrestrial 

 animal ; to animals such a habit is only possible in the 

 water, and in moving water at that, the latter serving to 

 transport the food. Physiologically the only sedentary 

 animals which are not aquatic, viz. the endo-parasites, are 

 really equivalent to aquatic animals in this respect, since 

 they are bathed on all sides in a nutrient medium. Without 

 exception, the larvse of sedentary animals are free-swimming, 

 as otherwise it would obviously not be possible to secure 

 distribution. 



The methods of achieving displacement on the shore are 

 very varied. Owing to the fact that water is relatively very 

 much denser than air a correspondingly lesser amount of 

 structural adaptation is necessary for an animal in the former 

 medium to raise itself from the ground. Consequently, there 

 are not a few bottom-living forms on the shore which are 



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