CHAP. XV Backboneless Animals 223 



calcareous, flinty, and horny. (a) The calcareous forms with 

 needles of lime have a world-wide distribution in the sea, from 

 between tide-marks to depths of 300 to 400 fathoms. They often 

 retain a cup - like form, but vary greatly in the complexity of their 

 canals. The sac-like Sycandra (or Grantia) comfressa is common 

 on British shores, (i) The siliceous sponges are more numerous, 

 diverse, and complicated, and the flinty needles or threads are often 

 combined with a fibrous " horny " skeleton. Venus'-Flower-Basket 

 (Euplectella) has a glassy skeleton of great beauty, Mermaids' 

 Gloves {Chalina oculata) with needles of flint and horny fibres is 

 often thrown up on the beach, the Crumb -of- Bread Sponge 

 (Halichondria paniced) spreads over the low -tide rocks. Some 

 have strange habits, witness Clione which bores holes in oyster 

 shells, or Suberites dofnuncula which clothes the outside of a whelk 

 or buckle shell tenanted by a hermit-Crab. Unique in habitat is 

 the freshwater sponge (Spongilla) common in some canals and lakes, 

 notable for plant-like greenness, and for the vicissitudes of its life- 

 history. (<;) The ' ' horny " sponges which have a fibrous skeleton 

 but no proper spicules are well represented by the bath -sponges 

 (Euspongia) which thrive well off Mediterranean coasts, where they 

 are farmed and even bedded out. 



Sponges are ancient but unprogressive animals. Their sedentary 

 habits, from which only the embryos for a short time escape, have 

 been fatal to further progress. They show tissues as it were in the 

 making. They are living thickets in which many small animals 

 play hide-and-seek. Burrowing worms often do them much harm, 

 but from many enemies they are protected by their skeletons and by 

 their bad taste. 



2. Stingittg- Animals or Coelenterata. — it is difficult to 

 find a convenient name for the jellyfish and zoophytes, sea-anemones 

 and corals, and many other beautiful animals which are called 

 Coelenterates ; but the fact that almost all have poisonous stinging 

 lassoes in some of their skin-cells suggests that which we now use. 



Representatives of the chief divisions may be sometimes found 

 in a pool by the shore. Ruddy sea-anemones, which some call 

 sea-roses, nestle in the nooks of the rocks ; floating in the pool and 

 throbbing gently is a jellyfish left by the tide ; fringing the rocks 

 are various zoophytes, or, if we construe the name backwards plant- 

 like animals ; beside.s these, and hardly visible in the clear water, 

 are minute translucent bells some of which have a strange relation- 

 ship with zoophytes ; and there are yet other exquisitely delicate, 

 slightly iridescent globes — the Ctenophores which move by comb- 

 like fringes of cilia. But we must search an inland pool to find 

 one of the very simplest members of this class — the freshwater 

 Hydra which hangs from the floating duckweed and other plants. 



