Sponges (Porifera). 5 I 



Sponges (Porifera). 



Although in the earlier part of the 17th century it was debated 

 whether sponges were plants or animals, close investigation soon 

 rendered undoubted their animal nature. It was early remarked 

 that "sponge" when burnt gave off a smell of burning hair or 

 horn, and exact analysis showed it to be nearly allied to these sub- 

 stances. This in itself gave reason to suppose that the chemistry 

 of their life was animal rather than vegetable. Though a living 

 sponge is fixed and apparently motionless, it was found that the holes 

 in its surafec are capable of opening and shutting, and that from 

 the larger of them, when open, there is usually a strong stream 

 of water issuing. This is compensated for by small entering streams 

 through other holes far more numerous but generally invisible 

 without magnification. Further it was found that the young 

 sponge (varying from microscopic size to that of a pin's head) 

 swims freely about by means of little waving hairs (flagella) over 

 its surface. Finally it was shown that sponges live on solid food. 

 While thus possessing all those characters that are more frequent 

 among animals than plants they never contain any traces of the 

 cottony and woody substances especially characteristic of the 

 vegetable kingdom (cellulose). 



The water entering by the small pores passes through a system 

 of branching and fine canals, and is collected again by a similar 

 system into the outflowing current from the large holes (oscula). 

 At the junction between the two systems of tubes are the most 

 vital organs of the sponge, little swollen cavities of microscopic 

 size walled in with tiny living particles, each bearing a vibrating 

 hair with which it lashes on the current and a transparent filmy 

 skirt, with which it catches the microscopical organisms of the 

 water which serve it as food. 



All this labyrinth of canals and cavities is living, soft flesh. 

 To prevent it falling a ready prey to the first hungry animal that 

 passes, it is set through and through with little flinty needles or 

 thorns, often of the most beautiful forms such as spears, anchors, 

 stars, marbles, hooks, bows, etc. A smaller group of sponges 

 has its spines of chalk, to serve the same end. A very large num- 

 ber of the flinty sponges cement their spines together with the 

 horny substance already referred to; a few have lost the fhnty 

 spicules entirely and, to withstand better the shocks of the waves, 

 have replaced them by the more elastic cement. The net-like 

 skeletons of this last small group form the sponges — bath-sponges, 

 toilet-sponges and the rest, with which we habitually associate 

 the name. The animals in which they were contained are killed 

 by exposure to the air, and then removed by repeated washing. 



