INVERTEBRATA. 377 



It may seem surprising that a fixed and relatively slow- 

 moving animal such as Hydra should feed upon the active 

 little Crustacea ("water-fleas ") that swim about in pond- 

 water, but such is the case. In its ordinary position Hydra 

 is in fact fishing for them with its extended tentacles. As 

 soon as a water-flea touches one of these tentacles it stops, 

 evidently paralyzed, and remains fixed to the tentacle. 

 (How it is stunned we shall see presently.) The Hydra 

 gradually contracts the tentacle so as to bring the crus- 

 tacean near its mouth, which lies in the centre of the 

 circlet of tentacles, at the summit of a little cone the 

 hypostome. The other tentacles help in forcing the prey 

 against the mouth, which then expands widely and seizes 

 it. An ordinary water-flea is thicker than a Hydra, and 

 when swallowed usually causes a prominent bulge in the 

 cylindrical body of its captor. It is easy to see that the 

 cavity in which the prey undergoes digestion extends for 

 the whole length of the cylinder, but ends blindly at the 

 attached end. There is no anus, the unassimilable parts of 

 the food being ejected at the mouth. 



2. General Structure. Examined under the microscope, 

 whole or in thin sections, further details of structure can be 

 made out. Hydra is multicellular, i.e. composed of many 

 cells, not of a single cell as are the Protozoa. These cells 

 are arranged in two very definite layers an outer one 

 called the ectoderm, and an inner one, the endoderm, which 

 forms a lining to the digestive cavity or enteron (fig. 195, A). 

 These two layers, as well as the cavity, are continued into 

 the tentacles, each of which is thus a miniature of the main 

 body (see fig. 194). Between these two layers of cells, every- 

 where, there is a layer called the mesoglcea which is a 

 secretion, and not a cell-layer. This is highly elastic and 

 performs the same function as the cuticle of Paramecium, 

 causing the body to revert to its normal form as soon as 

 contraction has ceased : it is in fact the skeleton of Hydra. 



3. The Ectoderm. The main part of the ectoderm 

 consists of cells resembling those of columnar epithelium, 

 tapering somewhat internally and ending in a process which 



