THE CLASSIFICATION OF ANIMALS 35 



the name of the group) through which water carrying the 

 food enters. The cells forming the outer layer of the sponge 

 are flattened, but, as we have already seen (p. 25), are capable, 

 in some cases at any rate, of changing their shape i.e. of con- 

 traction. The cells forming the inner layer in the simplest 

 sponges, and portions of the inner layer in all sponges, are very 

 peculiar and characteristic. They are termed collar-cells, and 

 each has a free end facing the cavity of the sponge, and a basal 

 end projecting into the jelly. Projecting from the free end there 

 is a flagellum, by the vibration of which the current of water 

 is produced which enters through the pores and brings food 

 to the animal. Surrounding the flagellum is a funnel-shaped 

 collar, apparently composed of stiff cytoplasm, inside which 

 the current apparently forms a little eddy, and in consequence 

 of this eddy food particles are directed inwards and engulfed 

 by the collar-cells. The common bath-sponge is the dried 

 and cleaned skeleton of one of the more complicated sponges. 

 In practically all sponges the skeleton consists of flinty or 

 calcareous needles in some these are bouud together into 

 ropes by a gelatinous substance termed spongin. In the bath- 

 sponge the ropes of spongin are there, but the needles or 

 spicules have disappeared. On this depends the usefulness 

 of the sponge, for were we to use an ordinary species of sponge 

 we should lacerate our faces every time we performed our 

 ablutions. 



The Ccelenterata a phylum which includes polyps, jelly- 

 fish, sea-anemones, and coral-forming animals have, like the 

 Porifera, only a single opening leading into the system of 

 tubes of which their bodies are built up ; but this opening 

 is used both for taking in food (ingestion) as well as for 

 ejecting the undigested material. The walls of the tubes 

 are, like those which make up the bodies of Porifera, com- 

 posed of two layers of cells, the outer of which is termed 

 ectoderm (i.e. outside skin) and the inner, endoderm (i.e. 

 inside skin), but the tubal walls are not perforated by pores, 

 nor does the inner layer consist of collar-cells, nor do the Ccelen- 

 terata obtain their food by causing currents in the water. 

 On the contrary, the terminal opening of each tube is used 

 for taking in food, and is termed the mouth. It is typically 

 surrounded by a ring of tentacles, by means of which prey 



