Tethya. 
ZOOPHYTA. SPONGIADi®. 
519 
of horny fibres, as the species so commonly used for domestic pur- 
poses ; or compound, being strengthened by calcareous or siliceous 
spicula. The gelatinous matter, abounding in transparent globules, 
connects the different parts of the skeleton, lines the various canals, 
and forms the margins of the openings. The 'pores are minute open- 
ings with a gelatinous niargin, strengthened or defended by the skele- 
ton or spicula, into which the water enters in currents, generated pro- 
bably hy a ciliary apparatus which has not yet been perceived by the 
micrcsi The water, after traversing the interior canals, is ejected 
by meai s V' ■ ■r?-fres, which are larger than the pores, and in many spe- 
cies are e!e i ' above the surface, in the form of perforated papillae. 
The ova are ijumerous, at first appearing like groups of minute, irre- 
gular-shaped, opaque granules, derived from the gelatinous matter, which 
unite into o liie bodies, falling at maturity into the canals, and expelled 
by the orifices. These ova float in the water, and exhibit spontaneous 
motion by tie rapid action of the ciliae, which cover the anterior por- 
tion of tl}e body, and at length attach themselves, and then expand in- 
to the formt of maturity. 
The ciirrents from the orifices are best exhibited by placing the re- 
cent aiiimri' in a shallow dish of water, and throwing a little powdered 
chalk on ti e surface, the motions of which will indicate the direction 
of the streams. For the purpose of examining the skeleton, it is requisite 
to Uaacerate the sponge in hot water, which removes the gelatinous mat- 
ter, and leaves the remainder in a state fit to be examined under a micro- 
scope. When the spicula are siliceous, the animal matter may be re- 
moved by nitric acid or by combustion, as was practised by Muller 
(Zool. Dan. t. Ixxxv.), when the vitreous needles will appear unaltered. 
In examining cabinet specimens of sponges, care must be taken to 
make allowance for the degree of maceration they may have under- 
gone, as, upon this circumstance, their porosity, friability, or softness 
depend. 
Gen. XLII. TETHYA. — Hemispherical, with numerous fibres 
radiating from the centre to the surface, and consisting of 
linear, fusiform, siliceous spicula. — -An intermediate ge- 
nus between Cydonium and Halichondria. 
91. T. cranium , — Surface regular and closely villous. 
Alcyonium cranium, Mull. Zool. Dan. t. Ixxxv. f. 1 — A. Lyncurium, 
Jameson,, Wern. Mem. i. 563.— Spongia pilosa, Mont. Wern. ii. 119, 
t. xiii. f. 1, 2. — Adheres to stones in deep water, Zetland, where it is 
termed Sea-Apple. 
