Tethya. ZOOPHYTA. SPONGIADiE. 519 



of horny fibres, as the species so commonly used for domestic pur- 

 poses ; or compound, being strengthened by calcai'eous or siliceous 

 spicula. The gelatinous matter, abounding in transparent globules, 

 connects the different parts of the skeleton, lines the vaiious canals, 

 and forms the margins of the openings. The pores are minute open- 

 ings with a gelatinous margin, strengthened or defended by the skele- 

 ton or spicula, into which the water enters in currents, generated pro- 

 bably by a ciliary apparatus which has not yet been perceived by the 

 microscope. The water, after traversing the interior canals, is ejected 

 by means of orifices, which are larger than the pores, and in many spe- 

 cies are elevated above the surface, in the form of perforated papillae. 

 The ova are numerous, at first appearing like gi-oups of minute, irre- 

 gular-shaped, opaque gi-anules, derived from the gelatinous matter, which 

 unite into ovate bodies, falling at maturity into the canals, and expelled 

 by the orifices. These ova float in the water, and exhibit spontaneous 

 motion by the rapid action of the ciliae, which cover the anterior por- 

 tion of the body, and at length attach themselves, and then expand in- 

 to the forms of maturity. 



The currents from the orifices are best exhibited by placing the re- 

 cent animal in a shallow dish of water, and throwing a little powdered 

 chalk on the surface, the motions of which will indicate the direction 

 of the streams. For the pui-pose of examining the skeleton, it is requisite 

 to macerate 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 HalicJtondria. 



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. 1 19, 

 t. xiii. f. 1, 2. — Adheres to stones in deep water, Zetland, where it is 

 termed Sea-Apple. 



