SPONGES. 



11 



SPONGES. 



The ordinary sponge of commerce gives a person no idea of what 

 a sponge is, since it is but the skeleton of the animal from which all 

 the soft parts have been removed ; and even 

 were the flesh present, the commercial form 

 would not be the best to begin the illustration 

 of the group, on account of its great complexity. 

 For this purpose it is better to take one of the 

 simpler forms, or even a diagram like that 

 shown, from which all unessential features have 

 been removed, remembering that all sponges are 

 found on essentially the same plan, only further 

 elaborated. 



In this simple diagram we have a great 

 advance over the Protozoa, in that the body is 

 composed of a multitude of cells. This form con- 

 sists of a vase-like body made up of two layers 

 of cells ; an outer layer of flattened forms (the 

 ectoderm) and an inner layer of larger flagellated 

 cells, each with a collar. These are the entoderm, or digestive cells. The 

 vase is open at the top, while through the sides are several openings. 

 The flagella of the entoderm are in constant motion, creating a current 

 of water which passes, in the direction shown by the arrows, into the 

 central chamber through the lateral openings and out through the one 

 at the top. It is from the water thus constantly drawn through the 

 sponge that the food is obtained, and this is eaten and digested by 

 the entoderm cells in exactly the same way as by the single cells of 

 the Protozoa. 



From this simple type we may derive any sponge by two processes. — 

 one a complication of the structure found here, the other the insertion of 

 a third layer '(the mesoderm) between the two. The complication consists 

 in a multiplication of the number of digestive cavities, and the production 

 of the lateral pores into longer canals which may further branch. This is 

 shown in the next figure, the same letters being used as before. 



Fig. 9. — Diagram of a simple sponge. 

 a, ciliated cavity; d, digestive 

 cells ; e, excurrent canal ; i, incur- 

 rent canal; p, ectodermal cells. 

 Compare the digestive cells with 

 Fig. 8. 



