CONTINENTAL SHELF 235 



contains in suspension a quantity of organic and 

 mineral matter derived from the shore. 



Taken as a whole, the area from low-water mark to 

 the edge of the Continental Shelf possesses a fauna 

 which is perhaps richer and more varied than that of 

 any other area on the surface of the globe. A cata- 

 logue of its inhabitants would include all the chief types 

 of invertebrate animals (except the air-breathing 

 Arthropods) and many fishes. It would be impossible 

 within the space at our disposal to attempt even an out- 

 line sketch of this fauna and of its variations in different 

 parts of the world, but for the guidance of those un- 

 familiar with zoology a few of the commoner types 

 represented in the illustrations may be mentioned. 



Sponges (Porifera) often form a conspicuous part of 

 the contents of the dredge or trawl in waters of moderate 

 depth, and deserve the attention of the collector, not 

 only for their own sake, but also on account of the 

 numerous Crustacea, worms, and other animals which 

 burrow in them or shelter in the crevices. In passing 

 from shallow to deeper water the shapeless encrusting 

 sponges of the littoral region are replaced, in part at 

 least, by types of definite and often beautiful form 

 (Fig. 165). This is especially the case with the glass- 

 sponges (Hexactinellida), of which the well-known 

 Venus's Flower-basket is an example. 



Below the depth at which true seaweeds flourish the 

 dredge is often filled with masses of what the fisherman 

 or -trawler terms " weed," but which the zoologist 

 recognizes as plant-like animals. Even for the zoologist 

 it is not always easy to distinguish by the unaided eye 

 between the feathery tufts of Hydroids (Figs. 90, 91, 

 167 ) and the much more highly organized Polyzoa. Be- 

 longing to the same division of the animal kingdom 

 (Coelenterata) as the Hydroids are the sea-pens, or 



