532 
SPONGES. 
interior burst out and grow over the outer flagellate layer of cells, and the latter 
become the collar-cells of the adult sponge. A minute sponge with one oscule 
results from the development of the fertilised ovum. An extensive crust with 
numerous oscules may be regarded either as a colony in which each oscule 
represents an individual, or simply as one individual 
in which the growth of the body necessitates the 
formation of new channels for the conveyance of 
food materials. 
We are now in a position to answer the question, 
What is a sponge ? It is obviously a living animal 
organism. The next question — the position of 
sponges in the animal kingdom—is not settled. All 
are now agreed that sponges come somewhere 
between the Protozoa and the rest of the animal 
kingdom, or Metazoa. It is accepted that sponges 
cannot be classed with Protozoa, for the embryo 
consists of definite groups of cells giving rise to 
distinct tissues. Some zoologists class sponges with 
the Coelenterata. The opinion here adopted is that sponges form a special 
subkingdom—Porifera. 
The bread-crumb sponge lias been selected as an example for explaining the 
nature of sponges, because of the historical facts associated with it, and its 
occurrence round the British coasts. Its anatomy is, however, somewhat com¬ 
plicated ; but there are other simple forms, the study of whose structure renders 
it possible to trace the path of development along which the more complex forms 
have proceeded. One of the simplest of sponges 
is Ascetta primordialis, found on seaweeds 
in the Mediterranean; in its simple unbranched 
condition it forms a minute white sac about a 
twenty - fifth of an inch in height, opening 
perforated by pores, through which water 
passes into the interior. The walls of the sac 
are composed of two layers, an inner lining of 
collar-cells, and an outer layer, consisting of a 
gelatinous matrix containing amoeboid cells and 
transparent three-rayed spicules of carbonate 
of lime. A canal-system can hardly be said to 
have arisen since the walls of Ascetta are thin 
and not folded. The spicules support the walls 
and serve as a framework for the pores. By 
eliminating the spicular skeleton, and by supposing the tube or vase to be 
more globular, we obtain the “ olynthus - form ” which has been regarded as the 
hypothetical ancestor of all sponges. A canal-system arises when the walls grow 
thick or form folds or give ofl‘ pouches or tubes. The folds or pouches may 
above by a wide round oscule, and narrowing 
below to a stalk. The walls are very thin and 
AN ASCON SPONGE. 
Magnified 20 times; B, 80 times; C, 
Transverse section ; />, Collar cells. (Mag¬ 
nified 700 times.) 
SECTION OP WALL OP VENUS’ FLOWER- 
BASKET, SHOWING SOFT PARTS.— 
After F, E. Scliulze. 
