INTHODTJCTTOX. 



the inner surface of the Ascon-sac, confine themselves to these diverticula, and 

 are replaced in the central cavity, into which the diverticula open, by flat euto- 

 dermal epithelial cells, which are similar in structure to the ectodermal epithelial 

 cells on the outer surface. The diverticula, which alone in these higher Sponges 

 are clothed with collar-cells, are termed ciliated chambers. The afferent pores, 

 scattered over the whole of the surface of the Ascon-sac, confine themselves to 

 the ciliated chambers. The central cavity communicates with the outer water 

 by one aperture only, the terminal osculum. 



A further differentiation is produced by the development of a special dermal 

 lamella, which covers the outer surface. This lamella — the skin — is perforated 

 by numerous small holes, the inhalant pores. In the Hexactinellida and the 

 Hexaceratina the pores lead into wide cavities which are often traversed by 

 slender trabecula>. These suhdermal cavities commwaiQfxie with the chambers by 

 numerous small holes in the chamber-wall — the afferent pores of the chambers. 



In the other Sponges — the higher Calcarea, the Chondrospongioe, and the 

 Cornacuspongite — the layer of ciliated chambers is folded in a more complicated 

 manner than in the two orders Hexactinellida and Hexaceratina ; and the 

 intervening space is filled with mesogloea, which develops to such an extent 

 that only comparatively narrow caiials are left open. These canals lead on 

 the one hand from the inhalant pores in the skin down to the ciliated chambers, 

 and on the other hand from the chambers to the central, often small, cavity 

 which opens out by the osculum. 



These canals are much branched and form two systems — the inhalant and 

 exhcdant canal-system. 



The ciliated chambers are in the Syconidao and Sylleibidae, among the higher 

 Calcarea, and in the Hexactinellida and Hexaceratina, large, oval or sac- 

 shaped ; and in the other Sponges smaller, spherical or oval. 



Most Sponges have a skeleton. The few which are destitute of any support 

 may be regarded as rudimentary forms. 



The skeleton consists in the Calcarea of spicules composed chiefly of 

 carbonate of lime. In the Hexactinellida and Chondrospongiae the skeleton 

 consists of spicules composed chiefly of silica, which appears in the amorphous 

 state and may be compared to opal. In the Cornacuspongia? we find originally 

 a skeleton composed of monaxon siliceous spicules. These are attached to each 

 other by a cement of a horn- or silk-like chemical nature, which is known as 

 spongin. In a great number of the Cornacuspongise the quantity of spongin 

 greatly increases, and a skeleton is formed which consists of a network of 

 spongin-fibres, in the axis of which proper spicules are contained. Finally, 

 in the Horny Sponges the spicules disappear altogether, and the skeleton 

 consists of spongin in which foreign bodies (sand &c.) are often embedded. 



All the Sponges the development of which is known undergo a complete 

 metamorphosis. The ova and spermatozoa are similar to those of other Metazoa. 

 The sexes are often distinguished ; but some Sponges are certainly hermaphro- 

 dite. The ovum is fructified in the body of the mother, and passes through the 



