THE SPONGES 



3507 



SECTION OF WALI, OF VENUS'S 

 FLOWER BASKET, SHOWING 



SOFT PARTS. After F. E. 

 Schulze. 



them daily, the embryos will be observed being driven out of the oscules, and swim- 

 ming about with the broad end forward. After from twenty-four to forty-eight 

 hours of independent roving existence, an embryo fixes itself by its broad end, and 

 becomes flattened. By a remarkable transformation peculiar to sponges, the large 

 granular cells of the 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 fertilized 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 neces- 

 sitates the formation of new channels for the con- 

 veyance 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 Crelenterata. The opinion here adopted 

 is that sponges form a special subkingdom Porifera. 



The bread-crumb sponge has been selected as an example for explaining the 

 nature of sponges, because of the historical facts associated with it, and its occur- 

 rence round the British coasts. Its anatomy is, however, somewhat complicated; 

 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 pro- 

 ceeded. One of the simplest of sponges is 

 Ascetta primordialis , found on seaweeds in the 

 Mediterranean; in its simple unbranched condi- 

 tion 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 



AN ASCON SPONGE. 



A. Magnified 20 times; B. 80 times; C. 

 Collar celsl. 



Transverse section ; 

 (Magnified 700 times.) 



D, 



