THE BEXTHAL 199 



spicules or stinging cells and their powers of reproduction and regenera- 

 tion. Hydroid polyps disappear as one leaves the coast and as the 

 water deepens, 23 since in the quiet water of the deeps all available 

 solid support is covered with ooze. Many tubicolous worms, especially 

 those with calcareous tubes, require a solid basis. Bryozoa are pre- 

 dominantly rock dwellers; sand is not favorable for them, but their 

 small size enables them to gain a foothold on the leaves of plants, 

 shells of mollusks, or the carapaces of crustaceans. The great majority 

 of simple and compound tunicates inhabit rock bottom. 



Many of these sessile animals have the power of reproducing by 

 budding, forming colonies of numerous individuals. Broad areas of a 

 rock may thus be covered by a single species of animal. Hydroid polyps 

 form miniature forests on stones and rocks; the beautiful orange coral, 

 Astroides, covers the rocks just below low tide mark in the Mediter- 

 ranean. The green and red sea anemones, Actinia equina, make a gaudy 

 carpet on the rocks of Helgoland. Tunicates often occur in similarly 

 thick masses. Tubicolous worms may cover wide areas; Serpula tubes 

 may form a tangled web among which the gill-funnels appear like col- 

 ored flowers; the leathery tubes of Sabellaria cover low rocks close to the 

 surface in the North Atlantic and Mediterranean so thickly that the 

 openings of the tubes are juxtaposed, and the surface appears like 

 that of a honeycomb. 



Of the mollusks, snails and chitons predominate on rocks, and 

 lamellibranchs in muddy sand. The broad foot of snails functions 

 better on solid material. Actinians, with their creeping foot, also are 

 more numerous on rock. Echinoderms, with their ambulacral feet pro- 

 vided with sucking disks, are, in general, common on the rock habitat. 

 Those that live on sand are adapted to it by changes in their am- 

 bulacral feet. Lamellibranchs which live on rock usually have no 

 siphons, while their foot is provided with a byssal gland whose hard- 

 ened secretion forms a fibrous attachment. The foot may be reduced 

 in the rock-dwelling forms, never in the sand dwellers. 



Some animals living on rock bottom have adopted the mode of pro- 

 tection furnished by boring into the substratum, so common among 

 the sand inhabitants. Since this requires an enormous amount of work, 

 relatively few animals have been able to acquire this habit, and these 

 inhabit mainly limestone or tufa, or similar softer rock. Nevertheless, 

 such rock-boring forms are found in a large number of animal groups. 

 There are boring sponges ; boring annelid worms from both Chaetopoda 

 and Sipunculoidea; 24 the boring lamellibranchs are well known and 

 are derived from diverse families, thus Petricola and Venerupis 

 (Veneridae) ; Saxicava and Gastrochaena (Myidae) ; Pholas and the 



