16 FOULING OF ships' BOTTOMS. 



individual tiers may consist of from fom* to twenty longish cells 

 (Fig. 34). The walls of the cells are usuaUy thick. Sometimes some 

 of the branches are irregularly swollen and harbour a number of 

 very deeply pigmented cells, often in groups of fours ; these are the 

 reproductive cells of Polysiphonia. 



(c) Animals. 



When an acorn barnacle is scraped off the hull surface it generally 

 leaves a circular plate of lime still attached to the paint (Figs. 16 

 and 24). There is, however, one, type of fouhng organism which 

 rather closely resembles a barnacle base and should be carefully 

 distinguished from it. This is known as a Polyzoan and produces 

 another form of " coral patch " on the surface (Figs. 15 and 23). 

 The Polyzoan encrustation has a fine net-Mke or honeycomb structure 

 composed of numerous small cells, while a barnacle base resembles a 

 fish scale. 



Polyzoa also occur as flexible branching forms (Fig. 44), as well 

 as in the " coral patch " form. They are difficult to distinguish from 

 plant and hydroid growth, though their fine hpneycomb structure is 

 generally visible under a hand-lens (Fig. 45). 



Jelly-like masses, found usually on ships moored for a long time 

 in richly infested waters, may be sea-squirts (Figs. 19, 20 and 25) or 

 sea-anemones (Fig. 27). A thick leathery bag of jelly which ejects 

 water voluntarily or when compressed (sea-squirts), or a coloured or 

 white flattened blob of jelly, is tjrpical of such animals. 



Sponges, sometimes found under similar circumstances, are more 

 bag-like in structure with a rough surface. They have either a 

 single large opening to the bag or a very large number of such 

 openings from a colony (Figs. 19 and 26). 



pRisTKD IN Great Britain by Richard Clay and Company, Ltd. 

 Bungay, Suffolk. 



