386 THE INHABITANTS OF THE SEA, 
genus Grantia they consist of carbonate of lime. Though the 
skeleton of most sponges is formed both of horny fibres and 
of mineral spicules yet the proportions of these two component 
parts vary considerably in different species. In the common 
sponge, for instance, the fibrous skeleton is almost entirely 
destitute of spicules, a circumstance to which it owes the 
flexibility and softness that render it so useful to man, while 
they predominate in the 
Halichondriz, and some- 
times even, as in the 
Grantiz, completely su- 
persede the horny fabric. 
On examining a sponge, 
the holes with which the 
substance is everywhere 
Minute portion of the surface of Tethea Cranium, ,° 
sane: spicula projecting beyorid the pierced may be seen to 
surface. be of two kinds; one 
of larger size than the rest, few in number, and opening into 
wide channels and tunnels which pierce the sponge through its 
centre; the other minute, extremely numerons, covering the 
So ? 
a cuigate Sona! ~~. SE 
att es Sa 
Halina papillaris. 
Currents passing inwards through the pores (a a), traversing the internal canals (6), and escaping 
by the larger vents (c,d). 
wide surface, and communicating with the innumerable branch- 
ing passages which make up the body of the skeleton. Through 
the smaller openings or pores the circumambient water freely 
enters the body of the sponge, passes through the smaller 
canals, and, ultimately reaching the larger set of vessels, is 
evolved through the larger apertures or oscula. Thus by a still 
mysterious agency (for the presence of cilia has as yet been 
