SPONGES (- PORIFERA). 
(Tank No. 21.) 
Although in the earlier part of the last century it was debated 
whether sponges were plants or animals, close investigation soon rendered 
their animal nature undoubted. 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 substances. 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 surface 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 in¬ 
visible 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 (see 
note p, 13) they never contain any traces of the cottony and woody 
substances especially characteristic of the vegetable kingdom. 
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 par¬ 
ticles each bearing a vibrating hair, with which it lashes on the current, and 
a transparent filmy skirt, with which it catches any food that may pass. 
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. A smaller 
group of sponges has its spines of chalk, to serve the same end. A very 
large number of the flinty sponges cement their spines together with the 
horny substance already referred to; a few have lost the flinty 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. 
