74 



LOWER INVERTEBRATES. 



When they are fully contracted they ajijiear as so many knobs or bosses on the distal 

 end of the body, and when fully cx]ianded, I have seen them three and even four 

 times the length of the fully elongated body. The tentacles are very sensitive, and if 

 touched by some foreign object in the water, they rapidly contract, and the body also 

 sharing in the contraction, the entire creature is withdrawn as much as possible from 

 the area of disturbance and danger. ILjdra has been observed in two or three rare 

 instances to move from place to place by standing on its head, so to speak, using its 

 tentacles as feet, by which it attaches itself, then it arches the body and attaches the 

 foot-disk, releases the tentacles, straightens the body to arch it again, and so hitches 

 along like a measuring-worm or geometrid larva. Another very peculiar form of loco- 

 motion is described by Marshall, of Leipzig, as seen by him in certain HydrcB found 

 in brackish water. In this case the Hydra lies upon one side, and uses two tubercles 

 as large, lobate, pseudopodial processes which give a creeping motion to the creature. 

 Every one who has watched ILjdra in aquaria has probably seen it creep or glide 

 slowly over the surface of a leaf or of the glass. It keeps its normal position, attached 

 by the foot-disk, but glides slowly, and with a very uniform motion, over the surface 

 to which it is attached ; much as a snail creeps, only with a much slower movement. 

 This power of changing place is due to the cells in the foot-disk. Watching under 

 a microscope, this part of Hydra, when it is in motion, it will be found that the 

 external cells throw out pseudopodial processes, which extend in the direction in which 

 the animal is travelling ; so that Hydra can move by pseudopodia as truly as Amceba 

 does. In the position which Hydra so often assumes, that of complete expansion 

 with the tentacles extended to their utmost, and forming a very large circle, its 

 chances for getting food in the well-populated, often semi-stagnant waters in which 

 it is so frequently found, are very great. Any luckless crustacean of small size, such 

 as Cypris or Dap/uiia, that hapjjens to strike against one of those delicate tentacles 

 is pretty sure to be used as food by the Hydra. The tentacle against which the crus- 

 tacean has touched, curls around him, and after a few struggles his limbs fall power- 

 less, and he acts as thougli it had been paralyzed. 



This peculiar paralyzing or stupefjdng effect is caused by the action of certain sting- 

 ing or cnidocells (also called lasso-cells), which are most abundant in the tentacles, 

 but are also found in other parts of the body. Each one coflsists of a comparatively 



large body-part, from which stretch away interiorly 

 one or more slender protoplasmic processes to con- 

 nect with a deeper laj'er of the body-wall ; on the 

 outer end of the cell is usually found a small proto- 

 plasmic process Avhich projects into the surrounding 

 water, but is too small to be seen with the unaided 

 eye ; this latter process is termed a cnidocil, and 

 probably receives and conveys stimuli from the ex- 

 ternal objects to the cnidocell ; within the body of 

 the cnidocell is the capsule, a more or less ovate 

 structure, consisting of an outer wall wliich is ]ier- 

 fect and complete, and an inner wall which is folded 

 in upon itself at one end to form a tube, which for a very short distance is of some 

 considerable diameter, and then decreases in size and forms a long, thread-like tube, 

 coiled up in the cavity of the capsule ; within the larger, shorter part of this tube, 

 attached to its wall, are a number of recurved hook-like processes which vary in 



Fig, 64 — .-/. cnidocells of Tubulana larynx; 

 Ji, cnidocells of Hydra viridls. 



