SPONGES, 



37 



the water is made to flow through the cells from without, 

 being discharged into the interior of the sack, and poured 

 out in streams from the orifices (oscula) which terminate 

 the angles of the Sponge.* 



This beautiful and interesting discovery leaves no doubt 

 of the animal nature of the Sponges, which is still further 

 confirmed by some interesting observations we had lately 

 an opportunity of making on the irritability of Halichon- 

 dria sanguined. 



" When carefully watched under a power of 70 diame- 

 ters, this brilliant species exhibits the following appear- 

 ances : — At first we discern an uneven surface with little 

 eminences here and there, like hills in an undulating coun- 

 try. A great number of very slender glassy rods project at 

 various angles from the surface, perfectly straight, equal 

 in thickness in every part, with blunt tips. Webs of the 

 investing membrane cling around the bases of these rods 

 (spicula), and are a little elevated with them. 



" Presently from one and another of the hillocks, a 

 round bladder is seen pushing out, which gradually length- 

 ens, until it becomes elliptical. It is composed of a clear 

 gelatinous membrane, excessively subtile, with a yellowish 

 granular film spread irregularly over its surface. Orifices 

 are now perceived in the rounded tip of the bladder, the 

 formation and increase of which are so very gradual, as to 



defy detection, except by the result These orifices 



slowly alter, increasing or diminishing ; sometimes a 

 minute one appears at the margin of a large one, aug- 

 menting at the expense of the latter, until the dividing 

 film stretches across, — a narrow straight isthmus between 



* Trr.ns. Micr. Soc. iii. 137. 



