IDENTIFICATION OF MARINE GROWTHS. 7 



in countless numbers, often closely packed together, within the 

 slime which they themselves produce (see Fig. 40). The contents of 

 the ceils are brown or oUve -yellow in colour. 



(b) Plants C Weeds "). 



The seaweeds that form the " grass " on the upper parts of the 

 underwater plating are of diverse kinds like the seaweeds found 

 in rock pools on our own shores. They may be green, light or dark 

 brown, bright red or purple in colour (Figs. 2 to 8), and may reach a 

 length of several inches. 



(c) Animals. 



The best-known of all the animal fouling organisms (Figs. 9 to 

 16) are the barnacles. The form of these, with their conical shells 

 adhering firmly to the hull surface, is almost too well-known to need 

 description. 



The common type of barnacle, the " acorn barnacle " (Figs. 12 

 and 21), is most usually encountered, but occasionally stalked or 

 " goose barnacles " occur. The latter are attached to the hull by 

 a thick muscular stalk up to 3-4 in. in length, at the end of which is 

 the body with its shell (Fig. 22), which may be inconspicuous. 



The shellfish (moUuscs) which foul ships are represented by the 

 mussels and oysters (Figs. 13 and 14). They are not likely to be 

 mistaken for any other forms, since the shell is always formed from 

 two halves called " valves." They frequently occur in large num- 

 bers on the various underwater gratings of the ship; when they 

 grow on the hull itself they show that the anti -fouling coating has 

 completely broken down, since they are very easily poisoned. 



Tubeworms, as their name suggests, appear as white or greyish 

 limy tubes, which may be much coiled, lying fiat against the surface 

 or sometimes projecting outwards from it (Figs. 11 and 17). They 

 often occur in patches on the hull surface (when they are frequently 

 but incorrectly called " coral pat<3hes "). They are of considerable 

 importance in the fouling of propeller blades (Fig. 18). Their very 

 long and firm attachment anchors them well even on a rapidly 

 rotating screw, where they may occur along almost the whole 

 length of the blade, causing a very serious reduction in the pro- 

 pulsive efficiency. They are fairly sensitive to poisonous paints, 

 and hence their settlement on the hull indicates poor anti-fouling 

 properties. 



All the above forms possess shells which remain on the hull even 

 when the animal is killed, e.g., hy steaming through a fresh- water 

 zone. Other animal forms without limy shells can occur either as 

 plant-like branehing growths {e.g., hydroids) or jelly-like soft-bodied 

 forms. 



Hydroids (Figs. 9 and 10) are often important fouling organisms, 

 since they are firmly attached and some are very resistant to anti- 



