150 AN INTRODUCTION TO ZOOLOGY 



radial symmetry, which is not only general in Coelenterata, but also 

 tends to be assumed by all animals adopting a fixed mode of life. 

 We cannot speak of anterior and posterior ends or dorsal and ventral 

 sides, but only of oral surface, i.e. on the side of the mouth, and 

 aboral, i.e. on the side opposite to the mouth. This type stands 

 strongly contrasted with that in ourselves and the frog, in which the 

 parts are only symmetrically disposed with regard to one plane 

 passing through the long axis, cutting the body into right and left 

 halves, which in consequence we term bilateral symmetry. 



Hydra can easily be kept under observation in water in a watch- 

 glass, and if some " water-fleas " (small Crustacea, Daphnids) be 

 added their capture can be watched. The tentacles are widely 

 spread out, forming a primitive kind of net, and sooner or later a 

 water-flea comes into contact with one of them. Immediately it 

 does so it comes to a standstill and all movement ceases, as if it were 

 paralysed, and it remains adhering to the tentacle. This gradually 

 shortens down and bends over so that the animal is brought to the 

 mouth, which enlarges and takes it in, removing it from the tentacle. 

 It is passed inside into the enteron, and for some time causes a 

 distinct swelling in the body of the Hydra. Here it is digested, and 

 later the shell and other indigestible residue are ejected through the 

 mouth. 



The animal is so small that its structure cannot be made out by 

 dissection, as in the case of the frog, and it has to be studied by means 

 of sections and by isolating the cells. A transverse section of 

 Hydra shows that the body wall is composed of two layers of cells. 

 The outer layer, or ectoderm, is thinner, and consists of a number of 

 cells tightly packed together, forming a very efficient covering for 

 the animal. These cells, as we shall see later, are more or less cone- 

 shaped, with their bases outwards, and so a series of spaces are left 

 between their inner ends. The interstices are not left empty, 

 however, but are filled with smaller cells. Inside the ectoderm, and 

 so forming the lining of the gastral cavity, is the second layer of 

 cells, the endoderm, or better, the entoderm. This is composed of 

 much larger, more columnar cells, but, like those of the ectoderm, 

 they are radially arranged. Between the ectoderm and entoderm 

 is a layer of structureless jelly-like substance, the mesoglea, forming 

 a sort of strengthening sheath, the supporting lamella. Cells may 

 occasionally be found in it, but it should be borne in mind that such 

 cells do not originate in it, they migrate into it from the other layers. 

 It cannot then be regarded as a cell layer, but simply as a sheet of 

 jelly secreted by the ectoderm and entoderm. In Hydra it remains 

 quite thin, but in the " jelly-fish " it is enormously thickened, form- 

 ing the main bulk of the body. This type of body wall, consisting of 



