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 

 SECTION OF WALL OF VENUS' FLOWER- cannot be c i asse d with Protozoa, for the embryo 



BASKET, SHOWING SOFT PABTS. . / 



After F. E. Schulze. consists or definite groups or 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- 

 above by a wide round oscule, and narrowing 

 below to a stalk. The walls are very thin and 

 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 off pouches or tubes. The folds or pouches may 



AN ASCON SPONGE. 



Magnified 20 times ; B, 80 times ; C, 

 Transverse section ; />, Collar cells. (Mag- 

 nified 700 times.) 



