64 



HA RD WICKE' S SCIENCE-GOSSIP. 



fringe the mouth are hollow, as is the body, linear 

 or slightly tapered, and roughened at intervals with 

 whorls of tubercles. They are filled in the interior 

 with an albuminous, oleaginous substance, and at 

 fixed places this substance swells out into nodules 

 arranged in a spiral line, of which more anon. 



If we now make a transverse section across the 

 body, we notice that there is the large central cavity 

 {archenteron or digestive cavity) before- mentioned, 

 and which is surrounded by a double circle of cells. 

 The outer layer is called the ectoderm, the inner the 

 endoderm; that is, outer skin and inner skin. Be- 

 tween the two there is a thin, apparently structure- 

 less, supporting membrane, known as the intermediate 

 layer. The cells of the ectoderm are of a curious 

 shape, something like a pear, the large end being ex- 

 ternal, and the pointed end touching the supporting 

 membrane. But it will be readily understood that 

 intercellular spaces must occur between these pointed 

 ends of the internal portion of the cells. These spaces 

 are filled up by a mass of smaller cells of no particular 

 shape, they being so crowded together that to retain 

 any primary shape would be impossible. The endo- 

 derm cells are large, very irregular in shape, and of 

 a granular appearance. One end of each cell touches 

 the intermediate layer, while the other is directly 

 within the archenteron. The outer portion of each 

 cell contains chlorophyll-coated globules thus giving 

 the green colour to the animal, from which is derived 

 its specific name. 



The digestion (or indigestion ?) of a Hydra is ex- 

 tremely interesting. The internal portions of the 

 endodermic cells are each provided wilh a long taper- 

 ing process called a flagellum, and, besides this, in 

 many cases the protoplasm is pushed out into the 

 archenteron, forming what are known as pseitdopodia 

 or false feet. Now, when the food is passed by the 

 tentacles into the mouth, and so into the cavity of 

 the body, it comes in contact with a digestive sub- 

 stance secreted by the endoderm. This digests, to a 

 certain extent, the contained food, leaving, however, 

 portions which it is unable to digest. These latter 

 it takes up by means of its pseudopodia, and draws 

 them into the endodermic cells. Here, it seems, 

 they meet with a more powerful digestive juice which 

 completely reduces them to a state of fine division, 

 and from where the nutriment is ultimately diffused 

 throughout the rest of the cells. But while all this 

 is going on the flagella never cease from agitating 

 the food, and distributing it about the archenteron, 

 so that every cell shares in the work ; and then 

 afterwards, as before remarked, excrementitious 

 matter seeks exit through the mouth," and, what is 

 more, finds it. Such a method of digestion as is 

 outlined above is termed intracellular digestion. 

 Before we go any farther, sections of the tentacles 

 should be made, when it will be seen that their 

 structure is homologous with the body, and of which 

 they are merely outgrowths. But connected with 



them are some peculiar organs which we must not 

 omit to notice. Distributed about each tentaculum 

 are what appear under the microscope little warts, 

 and embedded in their substance small bodies of a 

 shape akin to a soda-water bottle. Each one of 

 these minute organs is filled with fluid and is en- 

 circled at the neck inside by several barbs directed 

 backwards. Coiled up in the interior is a long 

 delicate filament, and at the top there is another one 

 projecting outwards, stiff, and apparently immovable. 

 These constitute the organs of defence, or, scientifi- 

 cally, nematocysts. The method of capture is this : a 

 small animal comes swimming by, and in so doing 

 accidentally collides with the extended tentacles, thus 

 touching one or more of the stiff hairs just spoken 

 of. Instantly the neck of the vessel is pushed out, 

 and not only one, but numbers transform their 

 tentacula into spiny masses, and thus the roughened 

 surface enables the prey to be grasped firmly while 

 the long thread does its deadly work. The fluid 

 injected by this latter is very poisonous, and of a 

 nature that paralyses the animal undergoing treat- 

 ment, so that the Hydra may the more easily get it 

 into its mouth, especially if the capture be a large 

 one, for oftentimes it will be found with a worm 

 struggling in its tentacles. 



But it is singular to note the animal's instinct of 

 rejecting unsuitable or harmful food. Should a grain 

 of sand or other foreign matter touch any of the 

 numerous short stiff hairs, before-mentioned, which 

 plentifully bestrew the surface of the tentacles, it 

 does not attempt to evert the long deadly filament, 

 for when one of these threads is everted it cannot be 

 drawn back again but drops off. 



As before stated, the Hydra can move slowly along 

 on its foot by the emittence of pseudopodia. The 

 ectodermic cells of the pedal disc secrete a clear, 

 tenacious gummy substance, possessed of great 

 adhesive powers. When the animal wishes to move 

 quickly it progresses after the manner of the common 

 looper caterpillars, e.g., by stretching out its body to 

 the utmost, and fixing itself in advance by its mouth 

 or tentacula, and then loosening its posterior hold. 

 Either a forward or retrograde motion can thus be 

 made. 



In a natural state the Hydra can reproduce itself by- 

 budding or gemmation. A small tubercle appears on 

 the animal's body, and gradually grows and expands 

 a set of tentacles, and becomes in all respects similar 

 to its parent, except in size. For some time it re- 

 mains attached, meanwhile growing and feeding. 

 At last its foothold has become so slender that a 

 slight effort frees it from its parent, though it is not 

 unusual for them while still attached to develop 

 others from their own surface. The sexual method 

 of multiplication may only take place during the 

 summer months. At this time the male organs 

 (testes) are found near the mouth, and consist of from 

 eight to twelve rounded prominences. Each one 



