LIFE IN PRIMARY PERIOD 205 



all the geological periods and are still plentiful in the depths 

 of the Atlantic and in the less profound regions of the Philli- 

 pine and Japanese waters where they attain a considerable 

 size. The exactly rectangular arrangement of the six branches 

 of the large spicules, constituting the essential part of the 

 skeleton, gives it the appearance of elegant opaline lace. They 

 are generally vase-shaped, with the upper opening protected by 

 a kind of operculum formed of siliceous tissue with delicate 

 meshes. Within the fugitive living tissue enveloping the 

 skeleton are regularly arranged delicate thimble-shaped sacs 

 whose walls are covered with large cells each bearing a vibratile 

 flagellum arising from the bottom of a funnel. These are the 

 active elements, the choanocytes found in all Sponges, and which 

 so exactly reproduce the form of the remarkable Infusoria 

 of the Choanofl agellate order that James Clarke classed Sponges 

 as simple associations or colonies of these Infusoria. 1 It is not 

 impossible that the earliest Sponges were gradually formed by 

 an association of this kind, in which diverse elements were 

 afterwards differentiated. As a matter of fact, the Choano- 

 rlagellates frequently form both ramified and compact colonies, 2 

 and one of the last has even been called Protospongia. 



The ovoid sacs of the Hexactinellidae are found also in 

 the Hexaceratinae, which also have six-branched spicules 

 made of spongin, the elastic and flexible substance seen in 

 the fibres of toilet Sponges. These sacs were the origin of 

 the flagellate chambers of other Sponges. They are always 

 connected with a system of tubes which bring to them the 

 water that has been attracted by their flagella, and which 

 then passes into the efferent cavities opening on the outside 

 by means of large orifices, the oscula. To the Hexactinellidae 

 must be added, somewhat later, Sponges with calcareous 

 spicules, belonging to the Pharetronid family, 3 and then the 

 Stony Sponges with spicules united by a siliceous glaze, the 

 Lithistidae, 4 and finally the sponges with fundamentally four- 

 branched spicules (Tetractinellidae) or with spicules in the 

 form of a pin (Monactinellidae). From the last are derived 

 fibrous sponges without spicules, employed for domestic 

 purposes. But the organization of Sponges has remained 



1 LXIV. 



2 Salpingceca, Codosiga, Codonocladium, etc. 



3 From the Trias onward. 4 In the Jurassic. 



