282 THE MICROSCOPICAL NEWS. 
they must enjoy an equal immunity from the attacks of other 
creatures who would otherwise prey upon them, for a mouthful of 
the spicules would by most animals be relished about as much as 
would a mouthful of their descriptive terms by the average mortal. 
In one order of sponges—the Calcispongize—the skeleton consists 
entirely of interlaced spicule of calcic carbonate. In another 
order—the Fibrospongiz, a fibrous keratode skeleton is always 
present, and in most forms siliceous spicule are abundantly 
distributed, not only through the keratode, but in the sponge 
flesh, Halichondria, a section of which is issued with this num- 
ber, belongs to this order, and shows well the fasciculi of 
acicular spicules. The sponges of commerce are those forms of 
horny sponges which contain no spicule. In other sponges, such 
as the beautiful Euplectella, the spicules are very abundant and 
very closely united, and the quantity of keratode is excessively 
small. Finally, in Clionia there is no trace of keratode, and the 
skeleton is entirely siliceous. 
The reproduction of sponges is effected by a sexual process. 
Nucleated cells, exactly resembling the ova of higher animals, are 
formed by certain of the sarcoids becoming detached, and acquiring 
a spherical form while retaining their nuclei and nucleoli, while 
certain other sarcoids undergo changes resulting in the breaking 
up of their contents into numerous minute bodies—sfermatozoa, 
provided with long vibratile filaments, by which they can propel 
themselves through the water. Fertilization is brought about by 
the contact of one or more spermatozoa with the ovum, the interior 
of which then breaks up into two portions which sub-divide again 
and again, until the originally single cell comes to consist of a 
hollow oval chamber whose walls are composed of two layers of 
cells—an inner—endoderm, and an outer—ectoderm, At this stage 
the embryo is free, and swims about by means of cilia with which 
the ectoderm is covered. One end of the embryo then turns in 
and converts it into a hollow sac, and the gastrula, as it is now 
called, attaches itself by the closed end to some object at the sea 
bottom, and loses the cilia of its ectoderm, the cells of which unite 
closely with each other to form the syzcytium. Pores appear here 
and there in the syncytium, through whjch inhalent currents are 
caused to set in by the cilia of the endoderm, and the water is 
discharged by the opening at the apex, which forms the single 
exhalent aperture or osculum. Before this period, however, 
spicules have made their appearance in the ectoderm, and young 
sponge has acquired a tolerably complete skeleton. Many of the 
Calcispongize remain permanently in this condition of a hollow 
chamber with thin walls and a single osculum, In the more com- 
plex sponges further development takes place mainly by the growth 
of the syncytium, whereby the endodermal cells become separated 
