3520 



THE SPONGES 



large oscules are visible. From the pores fine in-current canals pass down to the 

 flagellated chambers, and from the latter proceed the rootlets of the out-current 

 canal system. With a lens the spindle-shaped siliceous spicules of the skeleton can 

 be made out. They are about one-fiftieth of an inch in length and unite in bundles 

 which partly surround the canals, and are partly scattered irregularly in the ground 

 substance; with the naked eye the bristling points can be seen projecting from 

 the surface. If a specimen be examined in autumn, there will generally be found 

 crowding the meshes at the base of the crust a number of small yellow spheres, 

 about one-twelfth of an inch in diameter, known as gemmules. They possess a firm 

 shell, with a small circular pore at one spot covered only by thin membrane. A 

 gemmule is a kind of internal bud, and is capable of developing into a new sponge. 

 When the season unfavorable to the life of the sponge arrives, a number of wandering 

 cells collect together into a mass which becomes coated with a horny covering. 

 Outside this a layer of siliceous spicules is secreted. In Ephydatia these spicules 



are, from their peculiar shape, termed amphidiscs, two 

 toothed discs being united by an axle, the layer of am- 

 phidiscs being arranged with the axles vertical to the 

 surface of the gemmule. In the succeeding spring the 

 cellular mass in the interior bursts out through the pore, 

 and develops into a sponge. The gemmule spicules of 

 Euspongilla are shaped like curved needles pointed at 

 each end, and with a granular surface. Gemmules are 

 formed, but apparently only rarely, in a few marine 

 sponges, such as Cliona and Chalina oculata. These 

 bodies are formed also by the fresh-water Bryozoa. In 

 addition to this asexual or vegetative formation of gem- 

 mules, fresh-water sponges also form ova and sper- 

 matozoa. When the ova are fertilized they undergo 

 segmentation, and form oval ciliated embryos which are 

 about one-seventh of an inch in length, and are easily 

 to be seen swimming about in a glass vessel of water. They swim with the broad 

 end forward; the anterior upper half is dark and semitranslucent, the posterior 

 lower half glistening white and opaque. 



EMBRYO OF A FRESH-WATER 



SPONGE (magnified 100 

 diameters) . 



HORNY SPONGES Order CERATOSA 



In this group, of which ordinary toilet sponges furnish examples, the skeleton 

 is chiefly composed of fibres of a horny substance, termed spongin, and allied in 

 composition to silk. In the toilet sponges the fibres of the skeleton form a close 

 felt-like network of soft elastic texture; but some horny sponges are hard and 

 brittle, and others of the consistence of India rubber. In most of the group foreign 

 particles, such as grains of sand, or siliceous spicules of other sponges, are present 

 in the fibres; and in some the foreign bodies form a thick core covered with a thin 

 coating of spongin. Even in the softest toilet sponges foreign particles are included 



