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 
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. 
EMBRYO 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 Ianthella from North 
Australia belongs to this group; also Lnfforia 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 
