OBSERVATIONS ON POI.YZOA. 207 



coenoecial orifice above, and the process of invagination is 

 completed. 



The polypide in its exserted state is buoyed up and sus- 

 tained by the pressure of the fluids within. Consequently 

 when invaginated it displaces an equal bulk of these in 

 the closed coencecium, and their reaction, aided by the 

 contraction of the muscular endocyst, is sufficient to evagi- 

 nate the whole. 



The evagination begins with the relaxation of the sphinc- 

 ter, which permits the ends of' the tentacles to protrude. 

 These daintily feel about for the cause of the alarm, and, 

 if they fail to detect the proximity of an enemy, the whole 

 fascicle is cautiously pushed out, and the sentient threads 

 suddenly and confidently unfolded. 



The polyzoon reasons from the sense of touch inherent 

 in its tentacles, and cannot be induced to expose itself 

 above the cnencecium until thoroughly satisfied, by these 

 sensitive feelers, that no danger is to be apprehended. In 

 fact, these plantlike creatures, singly mere pouches with a 

 stomach hanging in the midst, exhibit greater nervous ac- 

 tivity and "animality," than we find among the more highly 

 organized Ascidia, or shell-covered Brachiopoda. 



PLUMATELLA. 



The species of this genus abound near the shores of our 

 ponds, close to the surface, and are generally in company 

 with Fredericella (PI. 8). They may be found attached 

 to the under sides of flat stones, or floating boards, but do 

 not usually seek the narrow, dark recesses in which Fred- 

 ericella often occurs. Better fitted to endure the sun's 

 rays, they may, occasionally, be seen in positions exposed 

 to their full influence. I have been so fortunate as to col- 

 lect specimens of P. Arethusa which were growing from 

 the ends of the long water grasses ; their tiny branches, and 

 living, crystalline flowers glittering in the light, and sway- 

 ing to and fro in the open current without protection from 

 the heat, even at midday. 



The ccencecium is dendritic as in Fredericella, but the 

 growth is generally more luxuriant, extending over larger 

 surfaces, and the coencecial cells are wider in proportion to 



