466 CHARACTERS OF INVERTEBRATE ANIMALS 



should be killed, stained, and imbedded in paraffin wax or some 

 similar substance. Extremely delicate slices of known thickness 

 can be cut by means of the instrument known as a microtome, in 

 which a sharp razor is mechanically drawn across the imbedded 

 specimen, this being held firmly by a suitable device. The slices, 

 when prepared, have to be cleared from paraffin and mounted on 

 a glass slide, in some transparent medium, such as glycerine or 

 Canada balsam. 



Specimens of Hydra are brown (e.g. Hydra fusca) or green 

 (Hydra viridis) (fig. 286) according to the species, but in any 

 case the form and structure are much the same. The colour of 

 the green kind is particularly interesting, since it is due to the 

 presence of a pigment (chlorophyll) which is characteristic of 

 ordinary plants, and, as we shall see later, plays a leading part 

 in their nutrition. The body of a fully -extended individual is 

 a hollow cylinder closed at one end, the foot, by which attach- 

 ment to a firm object is effected, while it is narrowed and open 

 at the other, the aperture being the mouth. A circlet of long 

 slender tentacles is situated quite close to the mouth, just where 

 the body begins to narrow. The large internal digestive cavity 

 has no other opening to the exterior. We find here a perfect 

 example of the star-like or radial symmetry which is exemplified 

 in a less complete manner by a star-fish or regular sea-urchin, 

 and there is absolutely no trace of the bilateral symmetry so 

 characteristic of higher forms (see p. 21). In other words, there 

 is no distinction between front and back ends, right and left sides, 

 or dorsal and ventral surfaces. 



If a fully-extended Hydra be touched or shaken it will at once 

 become retracted, its body shortening into a little rounded lump, 

 while the tentacles become minute knobs. Such an arrangement 

 is obviously of a protective character. When in an extended 

 position the animal is on the look-out for prey, if such an ex- 

 pression may be used in the absence of visual organs. Its food 

 consists of small aquatic animals such as Water-Fleas (see p. 422), 

 which are much higher in the scale, and would at first sight appear 

 to have every chance of escaping capture. This, however, is 

 not the case, for if one of these active creatures happens to come 

 into contact with one of the tentacles of the Hydra it stops dead 

 as if paralysed, and is then by the help of the other tentacles 

 drawn down to the dilatable mouth and passes out of view into 



