PARASITISM, ETC., ON THE SEA-SHORE 149 



and Pleurobranchus and from the cephalopod Sepiola atlanttca, 

 but these cannot be called common. The same remark Is 

 true of the intermediate stage of a Tetrarhynchus found in 

 Nudibranchs. 



Nematodes or thread-worms occur in numbers on the 

 sea-shore. They may be free in the substratum or parasitic, 

 both on shore weeds and animals. A small thread- worm 

 is responsible for the small growth irregularities or galls 

 frequently seen on Ascophyllum. All fishes, shore forms 

 included, are likely to be parasitised by Nematodes, which 

 occur either in the gut cavity or in the wall of the gut or in 

 the body cavity. For the most part shore Nematodes are 

 of small size and unlikely to attract the interest of the 

 ordinary observer. A very large form, however, Icthyonema 

 grayi sometimes occurs within the test of the edible sea- 

 urchin Echinus esculentus, almost filling the cavity. 



Symbiosis. — This term is applied, in the strict sense, to 

 the close physiological association of two distinct organisms 

 for mutual benefit. So intimate, indeed, is the symbiotic 

 relationship that one or both of the partners may be incapable 

 of living alone. The most familiar case of symbiosis is 

 that of the lichens which occur so frequently on trees, on 

 rocks above high-tide mark, and elsewhere. As is well 

 known, every lichen is the outcome of a partnership between 

 a fungus and an alga, the latter supplying the chlorophyll 

 and the fungus forming, as it were, the matrix weaving 

 the separate algal cells together into a single, complex 

 entity. 



Another illustration is provided by the mycorrhizae of 

 trees, an intimate association between the tissues of plant- 

 roots (particularly those growing in humus), and the mycelium 

 of a fungus. The fungus decomposes organic substances 

 present in the soil and hands them on in assimilable form to 

 its partner, profiting, in return, by the protection and safety 

 from drought which the higher plant affords. The reason 

 why certain plants, such as heaths, are difficult to transplant 

 is that the partnership, owing to the disturbance of the soil, 

 has been temporarily destroyed. 



