P0R1FEEA : — SPONGES . 



527 



_ 405. Porifera. — Although the tribe of Sponges has been ban- 

 died from tbe Animal to the Vegetable kingdom, and back again, 

 several times in succession, yet its claim to a place among the 

 Protozoa may now be considered as pretty certainly determined, 

 by the information derived from Microscopic examination of its 

 minute structure. For the skeleton of the living Sponge, usually 

 composed of a fibrous network strengthened by spicules of mineral 

 matter, is clothed with a soft flesh ; and this flesh has been found 

 by Dujardin and all subsequent observers to consist of an aggre- 

 gation of Amceba-like bodies (Fig. 271, b), some of which (as Dr. 



Fig. 271. 



Structure of Grantia compressa: — a, portion moderately mag- 

 nified, showing general arrangement of Triradiate Spicules and 

 intervening tissue ; — b, small portion highly magnified, showing 

 Ciliated Cells. 



Dobie was the first to show*) are furnished with one or more long 

 cilia, closely resembling those of Yolvox (Plate ix., fig. 9), by the 

 agency of which a current of water is kept-up through the pas- 

 sages and canals excavated in the substance of the mass. And 

 from the observations of Mr. Carterf upon the early development 

 of Sponges, it appears that they begin life as solitary Amoebce; and 

 that it is only in the midst of aggregations formed by the multi- 

 plication of these, that the characteristic /S/30?igre-structure makes 

 its appearance, the formation of Spicules being the first indication 

 of such organization. The ciliated cells seem usually to form 



* " Goodsir's Annals of Anatomy and Physiology," No. 2, May, 1852. 

 See also Bowerbank, in "Transact, of Microsc. Society," First Series, 

 Vol. iii. (1852), p. 137. 



t "Annals of Natural History," Second Series, Vol. iv. (1849), p. 81. 



