SPONGES 



during which it swims freely by means of cilia, the sponge passes its 

 whole subsequent existence fixed, except in a very few instances, to 

 some foreign object. The attachment may be direct, the base of the 

 sponge being in contact with the substratum, or indirect, that is to 

 say, by means of a root tuft of long spicules which serve to anchor it 

 as it were in the mud. The latter method is only found amongst 

 those forms, usually inhabitants of deep water, which live in mud 

 or ooze, and it is to be looked upon as a special adaptation to life 

 under such conditions. 



Direct attachment is a rule without exception amongst Calcarea 

 and is the most usual method in all sponges, being universal amongst 

 forms which inhabit shallow waters and ai*e subject to more or less 

 violent currents. The substratum to which the sponge is fixed may 

 be a rock or alga, or it may be some other animal such as a crab, 

 shell-fish, or tunicate. The adhesion is effected by the cells at the 

 point of attachment, which are glandular in nature, and in some cases 

 secrete a basal plate of spongin or some similar substance. The 

 portion of the sponge body which is in contact with the substratum 

 may be diuwn out into a stalk or peduncle, often of considerable 

 length, by which the sponge is raised above its immediate surround- 

 ings (Figs. 8, 10, 11, 27, 37, and 38). In such forms the lower- 

 most portion of the stalk may be expanded into a foot or disc, 

 increasing the adhesive surface, or into root-like processes, as in the 

 fossil Ventriculites (Fig. 23). 



Rooting tufts of spicules are specially characteristic of the 

 order Li/ssacina of the Hexactinellids, where they are of very 

 frequent occurrence. They are also found in some Tetractinellids 

 (Fig. 2i), but are very rare in Monaxonida and are unknown in 

 Keratosa and Calcarea. 



Tlie instances, very few in number, in which the adult sponge 

 is not fixed in any way, are to be found amongst a few species from 

 deep Avater. The remarkable form, Disyringa, for instance (Fig. 

 26), lies loosely on the sea-bottom, and a similar state of things is 

 met with in some other Tetractinellids from the deep sea. In such 

 cases the weight of the body, loaded as it is with siliceous spicules, 

 is probably sufficient to prevent the sponge from being passively 

 transported hy the comparatively feeble currents to which it is 

 exposed. 



{h) Form and Growth. — The typical sponge form is that of a 

 hollow vase or sac (Fig. 1), attached by its base to some object. 

 At its upper extremity is a conspicuous opening, termed the osculum, 

 and the wall is perforated by numerous minute apertures, the 

 pores. During life water enters by the pores, and passes, either 

 directly or after a more or less tortuous course along a system of 

 canals in the' body wall, into the central space or gastral cavity, 

 to emerge by the osculum. 



