114 Guide to the Tnverielrata. 



Sponges, furnished with cilia and a frill-like extension or collar ; hence they 

 GALLEKY are termed ciliated or collar-cells. It is by the action of these 

 West Side ^^^^^ ^^'^^ ^ constant stream of water is propelled through the 

 Wall-cases spong*") thus furnishing it with food and the means of respiration. 

 7 & 8, The canal system of a sponge consists of incurrent and exourrent 

 cases" canals. The former open at the surface by small apertures or 

 11-16. pores, and conduct the water to small chambers lined by the 

 ciliated collar-cells ; these are connected with the excurrent canals, 

 which unite together and open by large apertures, or oscules, into 

 a central tubular cavity, or cloaca, or, in many instances, where 

 a cloaca is not present, direct at the exterior surface of the sponge. 

 The soft, fleshy structures are, in nearly all sponges, supported 

 by a hard skeleton formed in the cellular mesoderm or inter- 

 mediate layer of the sponge- animal ; and this skeleton may be 

 a framework of anastomosing horny fibres, or of siliceous or 

 calcareous spicules of microscopic dimensions, which are either 

 inclosed within horny fibres, or somewhat loosely arranged in the 

 soft tissues, or connected together in various ways to form a firm 

 skeletal meshwork. 



In the sponges with horny skeletons of resilient anastomosing 

 fibres, of which the most familiar representative is the ordinary 

 bath-sponge, no mineral particles or spicules have been formed, 

 and these sponges appear to have been incapable of being preserved 

 in the fossil condition, since none have as yet been discovered. 

 All the sponges exhibited in the cases belong either to the 

 Silicispongiae or to the Calcispongige, according as their skeletons 

 are of siliceous or calcareous spicules, and the former of these two 

 groups is by far the most numerous and important. 



1. — SiLicispoNGi^. In this division the spicular elements of the 

 skeleton are composed of amorphous or colloid silica, deposited in 

 delicate concentric layers around a central tube or axial canal, 

 which is usually open at one or both ends of the spicule. In 

 recent sponges the silica of the spicules is as clear and transparent 

 as glass, but it is seldom preserved in this state as fossil ; usually 

 the silica is now chalcedonic or entirely crystalline, or it has 

 become of a milky-white tint. In many spong( s, such as those 

 from the Upper Chalk of the South of England, the original 

 silica of the skeleton has now nearly entirely been replaced by 

 peroxide of iron or iron-rust, and the sponges appear in section 



