92 MANUAL OF ZOOLOGY. 



larva, the cells of the endoderm being ciliated (fig. 30, B, c), and enabling 

 the organism to swim actively through the water, whilst the cells of the 

 ectoderm are non-ciliated (fig. 30, B, b). In the process of growth, the 

 ciliated endodermal cells become gradually retracted into the interior of 

 the larva (fig. 30, C and D), till the body becomes completely invagtnated 

 upon itself. In this condition (fig. 30, E) it forms what Haeckel terms a 

 *' gastrula," and consists of two layers of cells, an outer and an inner, en- 

 closing a central cavity, which communicates with the outer water by a 

 single primitive opening. This aperture is formed by the invagination of 

 the body, and not by rupture of the walls of the central cavity. The 

 skeleton is formed in the ectodermal layer, and the primitive opening into 

 the body-cavity becomes finally effaced. In its further development, the 

 young sponge, now consisting of two cellular layers surrounding a closed 

 central cavity (fig. 30, F), fixes itself by one extremity to some foreign 

 object ; a primitive "osculum" is developed at the free extremity of the 

 larva ; and the walls become perforated with numerous small apertures, 

 which ultimately become the inhalant openings or "pores" of the adult. 

 It should be added that the account of the development of the Calci- 

 spongia given by Hseckel differs in some points very materially from the 

 above. 



It should be added, further, that according to the researches of Mr Saville 

 Kent, the so-called "ciliated embryo" of the sponges is not composed of 

 "cells," in the ordinary acceptation of this term, but is "a spherical or 

 ovate aggregation of typical collar-bearing Monads or spongozoa, con- 

 nected laterally and by their bases with one another, and with their 

 anterior flagellate collar-bearing extremity directed outwards." 



DISTRIBUTION OF SPONGES IN SPACE. Sponges are almost 

 exclusively marine, the Spongilla alone_being inhabitants of 

 fresh water ^and they are of almost universal occurrence. The 

 sponges of commerce are mostly obtained from the Grecian 

 Archipelago and the Bahama Islands. The common marine 

 sponges are mostly found attached to some solid object between 

 tide -marks or in deep water. One genus (Cliona) inhabits 

 branching cavities in shells, which the sponge excavates for 

 itself, apparently by means of its siliceous spicula ; and fossil 

 shells mined by a boring-sponge, allied to the recent Clionce^ 

 are found from the Silurian rocks upwards. The siliceous 

 sponges appear to be exclusively inhabitants of the deeper 

 parts of the ocean, and our knowledge of these beautiful forms 

 has been enormously increased during late years by the re- 

 searches into the fauna of the deep sea, which have been 

 carried out by Sir Wyville Thomson, Carpenter, Sars, and 

 other well-known observers. Much also has been added to 

 our knowledge of the Calcispongia by the elaborate investiga- 

 tions of Haeckel, Oscar Schmidt, Metschnikoff, and others. 

 The calcareous sponges are all marine, and all inhabitants of 

 shallow water, and the living forms are all of small size. 



DISTRIBUTION OF" SPONGES IN TIME. Remains of sponges 

 are known to occur in formations belonging to the Palaeozoic, 



