CLASSIFICA TION OF SPONGES. 1 1 7 



sponges of Britain. Mermaid's gloves, or Ckalina oculaia, is a large 

 branched form, with spicules mostly needle-like, and with a fibrous 

 skeleton as well. The "crumb of bread sponge," or Halichondria 

 panicea, grows on the rocks as a slightly greenish crust, broken at intervals 

 by crater-like exhalent apertures, with crowded needle-like spicules, 

 but without any fibrous skeleton ; it crumbles readily in our hands. On 

 the shore you often find oyster shells riddled with holes, as if they had 

 been bored all over with a gimlet ; these holes were tenanted by a 

 small yellowish burrowing sponge, Clione, which by some chemical action 

 eats into shells, and even into limestone rocks. Sometimes washed 

 ashore is the not less interesting Suberites domuncula, a compact orange- 

 coloured sponge of peculiar odour, which grows round a buckie-shell 

 tenanted by a hermit-crab, and thus gets carried about from place to 

 place — an unusual privilege for a sponge. But Suberites eats into the 

 shell of the buckie, and the hermit-crab doubtless leaves its quarters. 

 Unique in habitat are the fresh-water sponges (Spongilla) common in some 

 rivers, canals, and lakes, notable for their plant-like greenness, and for a 

 curious life-history which we shall afterwards relate. 



These flinty sponges are classified in part according to the spicules, 

 as their rays lie in one axis (Monaxonia), or in three (Triaxonia), or in 

 four (Tetraxonia), and F. E. Schulze has shown how each of these three 

 types of spicule is suited to, and indeed depends upon the soft structure 

 of the sponge in which it prevails. 



(3) Ceratospongia. — The so-called horny sponges, which have a frame- 

 work of spongin or sponge-stuff and no proper spicules, are well repres- 

 ented by the bath sponges (Euspongia), which thrive at many places 

 off the Mediterranean coasts. 



Life of Sponges. — The motor activity of a sponge is almost 

 restricted to the internal cilia. Sensitiveness to surrounding 

 influences is shown by the closure of the little pores, and 

 sometimes even of the large exhalent aperture. This is 

 effected by special muscle-cells in the mesoglcea, which are 

 probably stimulated by sensitive and nervous cells on the 

 surface. The food carried down the canals consists of 

 microscopic organisms and particles of organic ddbris ; these 

 are caught as they pass by the ciliated cells, which behave like 

 so many monads, swallowing first, digesting intra-cellularly 

 afterwards. From these cells which feed, surplus material 

 oozes to their neighbours, or is passed to wandering 

 amoeboid cells in the middle stratum. Useless debris is 

 rapidly got rid of also by the collared cells. Some of the 

 bright pigments, such as floridine, readily absorb oxygen, 

 and therefore help in respiration, while the green of the 

 fresh-water sponge is at least closely analogous to chlo- 

 rophyll. 



Sponges spread like plants by overgrowth or budding, and 



