TYPICAL GROUP. 



545 



When the season unfavourable 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 amphidiscs 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 

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

 matozoa. When the ova are fertilised they undergo EMBRYO 

 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 forwards ; the anterior upper half is dark and semitranslucent, the posterior 

 lower half glistening white and opaque. 



OF A FRESH - WATER 



SPONGE (magnified 100 diam- 

 eters). 



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 indiarubber. 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 in the main fibres. The large purple fan-shaped lanthella from North 

 Australia belongs to this group; also Lufforia archeri (Neptune's trumpet) 

 from Yucatan, forming a magnificent cornucopia, five feet in length. Darwinella 

 possesses peculiar horny spicules. 



Toilet- and The sponges commonly seen in shops may be arranged under 



Bath-Sponges, three species, all of which occur both in the Mediterranean and 



West Indies, namely, Euspongia officinalis, variety mollissima, the fine turkey- 



or toilet-sponge ; E. zimocca, the hard flat disc-shaped sponge ; and Hippospongia 



equina, the common bath-sponge or horse-sponge. Under these species are 



included a large number of " varieties " and " grades," classified according to form, 



quality of texture, colour, locality, etc. The sponge-merchant can define the exact 



locality whence a specimen came, by observing the presence of characters which the 



naturalist would not regard as specific. The merchant classifies his material into 



VOL. vi. 35 



