226 THE STUDY OF ANIMAL LIFE CHAP. 



Sponges may be classified according to their skeleton, 

 as calcareous, flinty, flinty and horny, and purely horn}'. 

 The calcareous forms with needles of lime have a world- 

 wide distribution in the sea, but not in the great deeps. 

 They often retain a cup-like form, but vary greatly in 

 the complexity of their canals. The purse-sponge 

 (Grantia compressa) is common on British shores. The 

 siliceous sponges are more numerous, diverse, and com- 

 plicated, and the flinty needles or threads are often 

 combined with a fibrous spongin 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 (Halichondriapanicea) spreads 

 over the low-tide rocks. Some have strange habits, witness 

 Cliona which bores holes in oyster shells, or Suberites 

 domuncula which clothes the outside of a whelk shell 

 tenanted by a hermit-crab. The purely ' horny ; 

 sponges which have a fibrous skeleton of " spongin ' 

 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 unpalatability. 



3. Stinging-Animals or Coelentera. It is difficult to 

 find a convenient name for the jelly-fish and zoophytes, 

 sea-anemones and corals, and many other beautiful 

 animals which are called Ccelenterates ; 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 



