PLUMATELLA STRICTA. 99 



small extent, as thin submerged stems, straws, &c., and as it continues to increase in size, the 

 branches having no extensive surface of attachment, soon become free, and a more or less 

 entangled bushy mass vpill be produced. These two variations are quite accidental, and 

 altogether the result of external agencies influencing the growth of the species ; indeed, the 

 very same specimen will often exhibit both at the same time, continuing closely adherent to 

 the stone or other object to which it happens to be attached, as long as this affords it sufficient 

 surface for adhesion, but becoming free, and assuming the form of the second variation as soon 

 as, in the progress of extension, it arrives at the margin of the sustaining body. 



In some specimens of P. repens the cells are short, and the orifices, consequently 

 approximated to one another, are placed at slight intervals along the branches, and give a sort 

 of moniliform appearance to the coenoecium ; in others, again, the cells are long, and the 

 orifices distant, and this condition chiefly occurs in the free variation where, the elongation of 

 the cells not being restrained by adherence to a fixed surface, the orifices become widely 

 separated from one another, and the moniliform character, so marked in the former condition, 

 is nearly lost. It is in the latter form that the transverse septa are most apparent, indeed, 

 I have met with specimens of the free variation of this species in which well-formed septa 

 occurred between almost every cell, while the elongated claviform figure of the cells increased 

 the resemblance to Paliidicella, and even rendered the specimen liable, at first sight, to be 

 mistaken for a large form of the latter Polyzoon. 



P. repens is a light-shunning animal, and will be found only on the under side of stones, 

 or in such other situations as are not exposed to the direct influence of daylight. The 

 polypides are timid, generally withdrawing on the least disturbance, and not again venturing to 

 display themselves till all is once more perfectly quiet. It often acquires a considerable size. 

 I have met with luxuriant specimens of the adherent variation radiating over a space of 

 upwards of twelve square inches on the surface of a flat stone, and specimens of the free 

 variation have occurred to me with the branches more than three inches in length. In Ireland, 

 P. repens is the most abundant and widely distributed of the fresh-water Polyzoa. It occurs 

 in greatest perfection during summer and autumn. 



2. Pluniatella stricta, Allman. 



Fie. 14.* 



* Figure copied from Van Beneden. 



