86 THE BIOLOGY OF THE SEA-SHORE 



species in exactly the same way as they would on an inorganic 

 substratum of rock. This may be a serious matter for the 

 species providing the foothold, its movements being impeded 

 or its growth stifled. It is advisable to give separate con- 

 sideration to this factor in the struggle for existence, as its 

 biological significance is of a diff^erent nature from that of 

 attack and defence as generally understood. 



Devices to secure Unimpeded Growth or Movement. — 

 (a) In Plants. — The broad fronds of algae such as Laminaria 

 offer particular advantages to encrusting animals, and to 

 adhesive forms of spawn, but in the majority of cases it is 

 probable that the functions of the plant are very little 

 incommoded thereby. Occasionally galls arc formed, but 

 not necessarily by encrusting forms. For instance, the 

 empty air- vesicles of Ascophyllum ?iodosum are often invaded 

 by the larvae of Mytihis edulis. Tobler (1909) has shown 

 that in such cases the growth of the larval shell causes the 

 plant tissue to split. The breaking away of parts of the 

 frond necessitates a strengthening of the mechanical tissues, 

 and this is provided for by a development of the medullary 

 hyphae. 



(6) In Animals. — The tendency of shore forms to assume 

 an encrusting habit has already been noticed (Chapter IV), 

 when it was shown to be of value in resisting wave-shock. 

 It is not difficult to see that in the crowded conditions of 

 shore life, any form offering a suitable surface is likely to 

 be encrusted or settled upon either by the same or by a 

 distinct species. Other factors than chance, however, are 

 concerned since, according to Harmer (1896), many species 

 of Polyzoa " show a decided or even exclusive preference 

 for particular situations ; as, for instance, species of Triticella 

 which are only found on certain Crustacea." Many encrust- 

 ing forms prefer the inside of dead shells of Pecten, Cyprina, 

 etc., to any other habitat ; Terehripora excavates tubular 

 cavities in the substance of the shells of Molluscs ; Hypo- 

 phorella inhabits passages which it forms in the walls of the 

 tubes of the Polychaets Lanice and Chcetopterus. One of 

 the Cheilostomata, Lepralia foliaceUy forms masses which may 



