128 ZOOPHYTES. 



tended four inches in length by three in breadth, which were the largest 

 I have observed. They are usually smaller, and there being frequently 

 much greater proportion of branches from one side of the stem, their for- 

 mation must be deemed irregular. — Figs. 1, 2. If a brancn bears several 

 twigs, the twig is very short, and often forks into two receptacles, with 

 hydrse. 



Here the hydra is of a greenish colour, and, as may be concluded, 

 completetely retractile within the tubular extremity. Its lunate head is 

 bordered with an outer and inner row of tentacula, in their circuit. I con- 

 jectured that the respective number in each might be possibly somewhat 

 enlarged or diminished, if the animal can alter the shape of its crescent. 

 The tentacula are clothed with very short active cilia, extremely difficult 

 to be detected, and only by interposing shades, in different positions, be- 

 tween the light and the object. 



As the number of tentacula certainly augments with age, it is impos- 

 sible to determine the true complement ; because it is impossible to dis- 

 cover which animals are of the longest survivance, and whether they have 

 grown under favourable conditions. In one of the hydra; from Blackball, 

 I enumerated (JO tentacula — in another 52. Those of the latter were so 

 disposed that 28 were in the exterior, and 24 in the interior row. In the 

 former, the inner row comprehended 22. I thought a large orifice visible 

 in the neck, under the head. 



The pulsation of a vessel below could be discovered, alternately 

 dilating and contracting about twelve times in a minute, when the ther- 

 mometer stood at G0° in the open air. By this alternate action, dark 

 particles were carried downwards and then returned. 



It is seldom that the cilia of such minute hydra; are visible, and in 

 these zoophytes they appeared more difficult to be found than usual. But, 

 by dropping a little muddy matter over the animals, they are roused to 

 great activity. Such an effect, produced by an extraneous substance 

 an the cilia, narrowly resembling the like, under similar circumstances, 

 on the Amphitrite ventUabrum, may render it doubtful whether the office 

 of these organs is restricted to those functions usually ascribed to them. 



Though manv animalcula were entangled among the tentacula of the 



