The Sea-mats or Polyzoa, represented by _Retepora_ (the Lace-coral) (Fig. 24) and _Flustra_ (the Sea-mat) of the present sea-shore, have a calcareous skeleton, or zoarium, which is easily preserved as a fossil. Polyzoa are very abundant in the Cainozoic beds of Australia, New Zealand, and elsewhere (Fig. 25). In the Mesozoic series, on the other hand, they are not so well represented; but in Europe and North America they play an important part in forming the Cretaceous and some Jurassic strata by the abundance of their remains. WORMS (VERMES).--The hard, calcareous tubes of Sea-worms, the Polychaeta ("many bristles") are often found in fossiliferous deposits, and sometimes form large masses, due to their gregarious habits of life; they also occur attached to shells such as oysters (Fig. 26). The burrows of the wandering worms are found in Silurian strata in Australia; and the sedentary forms likewise occur from the Devonian upwards. ECHINODERMATA.--Sea-urchins (Echinoidea) possess a hard, calcareous, many-plated test or covering and, when living are covered with spines (Fig. 27). Both the tests and spines are found fossil, the former sometimes whole when the sediment has been quietly thrown down upon them; but more frequently, as in the Shepherd's crown type (_Cidaris_), are found in disjointed plates, owing to the fact that current action, going on during entombment has caused the plates to separate. The spines are very rarely found attached to the test, more frequently being scattered through the marl or sandy clay in which the sea-urchins are buried. The best conditions for the preservation of this group is a marly limestone deposit, in which case the process of fossilisation would be tranquil (Fig. 28). [Illustration: =Fig. 26.--Fossil Worm Tubes (? Serpula.)= Attached to a Pecten. Slightly Enlarged. Cainozoic (Balcombian). Muddy Creek, Hamilton, Victoria. (_F. C. Coll._) ] [Illustration: =Fig. 27.= =A Regular Sea-Urchin (Strongylocentrotus erythrogrammus, Val.)= About 2/3 nat. size. Showing Spines attached. Living. Victoria. (_F. C. Coll._) ] [Illustration: =Fig. 28.--A Fossil Sea-Urchin (Linthia antiaustralis, Tate).= Test denuded of Spines. About 2/3 nat. size. Cainozoic (Janjukian): Curlewis, Victoria. (_Nat. Mus. Coll._) ] [Illustration: =Fig. 29.--Ophioderma egertoni, Broderip, sp.= About 1/2 nat. size. A Brittle Star from the Lias of Seaton, Devon, England. (_Nat. Mus. Coll._) ] The true Starfishes (Asteroidea), are either covered with calcareous plates, or the skin is hardened by rough tubercles; and these more lasting portions are preserved in rocks of all ages. The shape of the animal is also often preserved in an exquisite manner in beds of fine mud or clay.