THE SPONGES 33 



incurrent canal, passes through the openings in the walls com- 

 municating with the flagellate canal, along which it travels to 

 be expelled into, and finally discharged from, the large exhalent 

 canal ; and in this way a constant circulation of water is main- 

 tained throughout the Sponge. 



The body of the Sponge is composed of three layers of cells, of 

 which the outer layer is called the ectoderm, the inner layer the 

 endoderm, and the middle layer the mesoderm. The outer surface 

 of the Sponge and the walls of the incurrent canals are covered 

 by a single layer of flattened, scale-like cells composing the ecto- 

 derm layer, while the endoderm lining the exhalent or para- 

 gastric cavity is composed of cells very similar in shape. Not so, 

 however, the cells lining the radial or flagellate canals ; their 

 structure is quite different, and will require a higher magnification 

 to be clearly seen. They consist of a close continuous layer of 

 column-shaped cells, each terminating in a single long, slender 

 lasher (flagellum), surrounded at its base by a delicate, collar-like, 

 transparent upgrowth. 



These specialised cells, with which the radial canals are lined, 

 are of very great interest, because they are only to be found in 

 the Sponges, and not in the tissues of any other animal belonging 

 to the Metazoa. Each of these specialised cells, in addition to the 

 collar-like upward expansion and terminal flagellum, has a nucleus 

 and one or more vacuoles, so that it really very closely resembles a 

 miniature collared-monad (Choano flagellate) of the Infusoria. It 

 is by the constant movement of the flagella of these cells that the 

 water is drawn in from the incurrent canals, and apparently so 

 similar are the movements of the collar and lashers to those of 

 the collared-monads that particles of food are captured by those 

 cells in just the same way ; while the effete and insoluble matters 

 are swept onwards by the lashers and discharged through the 

 exhalent canal. 



In the light of recent research, the evidence seems to point 

 to the Sponges having developed from a collared-monad pro- 

 genitor, through some such ancestral type as the colonial In- 

 fusorian Proterospongia, described by the late Mr. Saville Kent. 

 The middle layer, or mesoderm, consists chiefly of a clear, jelly- 

 like, greyish mass in which lie the skeleton or spicule and spongin- 

 forming cells, and also certain curious cells called, from their 

 D 



