34 ZOOLOGY 



to go into the characters of all these phyla, but the general 

 features of the most important must be pointed out. We 

 begin with the two simplest, the Porifera or Sponges, and the 



Ccelenterata or Polyps. The 

 animals included in both these 

 groups consist of a series of 

 branched tubes whose walls are 

 made up of two layers of cells 

 separated from one another by 

 a secretion of semi-fluid consis- 

 tency known as the jelly. 

 This jelly often contains cells, 

 which have wandered inwards 

 from the outer layer, and which 

 in many cases secrete needles 

 of flinty or calcareous sub- 

 stance! This jelly, with the 

 cells which it contains, is at 

 once the simplest form of 

 skeleton, and the beginning 

 of the blood system. In 

 these groups the individual 

 is a little difficult to define. 

 In its simplest form it is a 

 single tube with a terminal 

 opening. When this tube 

 branches we regard this as 

 a kind of budding or asexual 

 FIG. 9. 4, diagrammatic longi- reproduction. The branches 

 tudinal section of a simple may separate completely from 

 Sponge. The collar-cells and the parent, but they may 

 spicules are represented in rem am attached to it and form 

 black; B, a single collar-cell . ,. . , , f , . , 

 enlarged ; col in A , collar-cell ; an individual of a higher order. 

 col in B, collar-cell; o, oscu- In both phyla one end of 

 lum ; P, pore ; spic, spicule. the tube opens directly or 



indirectly to the exterior by a 



wide opening. In the Porifera or Sponges this opening is 

 only used for casting out the undigested remnants of the 

 food, and is termed the osculum ; whilst the walls of the 

 tube are perforated by a multitude of minute pores (whence 



