THE BEXTHAL 181 



A very rich animal population develops in these thickets of seaweed. 

 Animals find in them hiding places, support, and abundant oxygen. 

 The seaweed supplies food for many forms, but less frequently from 

 its living tissue than from the organisms borne on its surfaces and the 

 detritus which originates from it and which is distributed by the move- 

 ments of the water through the neighboring area. 4 The inhabitants of 

 the seaweed masses not only attach themselves to the living and dead 

 leaves and stems, but also dig down among its roots, swim among the 

 plants, or creep upon the bottom. 



Common forms in this biotope are the small snails of the genera 

 Rissoa, Bittium, and Columbella, with other snails, which feed pri- 

 marily on the algal growth and slime which covers the living and dead 

 leaves of the weed. On this algal growth is a microfauna of protozoans, 

 nematodes, and copepods. The delicate branches of the hydroid polyps 

 attach themselves to leaves and stems, as do small sea anemones and 

 the curious sessile medusa, Lucernaria. Here also are to be found the 

 polyps of the sessile generation of the medusae Aurelia and Cyanea, 

 and both individual and colonial tunicates, such as Ciona and Botryl- 

 lus. Crustaceans live in great numbers in the seaweed beds, amphipods, 

 isopods, schizopods, and macrurans. Various types of worms maintain 

 themselves on and in the bottom among the seaweed roots. A few 

 lamellibranchs join this community, feeding directly on the seaweed 

 debris. With so many herbivores, the predators naturally are attracted 

 in numbers. The starfish Asterias feeds on various mollusks, and the 

 crab, Carcinides, seeks a variety of prey. The larger carnivorous snails, 

 such as Murex and Tritonium, appear in the Posidonia beds in the 

 Mediterranean. Numerous fishes depend on this fauna for their food. 

 Constant components of this biocoenosis are the pipefishes (such as 

 Nerophis and Syngnathus) , which resemble the eelgrass leaves in both 

 coloration and form, and are difficult to find. In the Mediterranean, in 

 the same situations, the related sea horses appear. Gobius, Blennius, 

 and the common stickleback are regular members. Lobsters and certain 

 crabs dig themselves burrows and live in them, like the conger eel. The 

 seaweed forms a breeding place for many fishes and for squids, as well 

 as feeding ground and refuge for juvenile fishes. 



Where no plants are available as protection and hiding place for 

 animal life in the eulittoral, a surprising number of animals of loose 

 bottom areas secure their positions by boring more or less deeply into 

 the sand, for the most part remaining fixed in position, though some 

 wander slowly about. Animals of very different organization have 

 become adapted to this mode of life and have acquired certain com- 

 mon characteristics. The great majority of these sand-dwellers feed 



