THE BENTHAL 201 



a sponge which exactly covers them; they are said to be protected by 

 its inedibility. Hermit crabs which live in snail shells to which sea 

 anemones are attached belong in this category, being protected by 

 the stinging cells of their messmates. Some sea urchins mask themselves 

 similarly; Toxopneustes brevispinosus bears on its upper surface a 

 number of shells, which are held fast by its tube feet, sometimes com- 

 pletely covering itself. 27, 28, 29 



As on sand bottom, plant growth begins at the low tide level in 

 rocky areas wherever the surf is not too violent. As there is no oppor- 

 tunity for roots to penetrate, this growth is limited to algae. The upper 

 levels, occasionally down to 10 m., are grown over with algae like 

 Sargassum and Fucus. Below these come the sometimes gigantic 



Fig. 34. — Dragonhead (Scorpaena porcus), one-third natural size. After "Aquarium 



neapolitanum." 



Laminaria, extending to a depth of 30-40 m. These may be extraor- 

 dinarily abundant and large. On the coast of Chile Macrocystis may 

 reach the surface from a depth of 80 m., and the growth may be com- 

 pared to that of a forest, as the mats of eelgrass compare with 

 meadows. 30 At other places, brown and red and bright-colored cal- 

 careous algae may form a mixed growth. 



These algal forests abound with animal life, even more than does 

 the more familiar eelgrass. Hydroid polyps, calcareous and siliceous 

 sponges, tubicolous worms, Bryozoa, and tunicates grow on the leaf-like 

 blades, as do small lamellibranchs with a byssal attachment. All sorts 

 of small creatures occur there, turbellarian, nemertine, and chaetopod 

 worms, snails and nudibranchs, small starfishes and sea urchins, and 

 numerous crustaceans and pycnogonids. A number of small fishes feed 

 here, and this is a normal habitat of the oyster. 



As on sandy bottom, the fauna of rock bottom is unevenly dis- 

 tributed. Stretches relatively close together may exhibit considerable 

 differences. Thus each of the coralline areas (Secche) of the rocky 

 bosses on the bottom of the Bay of Naples has its own peculiarities. 



