OBSERVATIONS ON POLYZOA. 221 



to meet with extraordinary power of adapting its organi- 

 zation to the different circumstances under wliich it might 

 l)e placed, and it does afford a happy illustration of this 

 principle. The colonies inhabiting a small brook near 

 Gorham, Me. (Young's Brook), has all three varieties. 

 The upper part of this stream where it is comparatively 

 narrow and deep and where the supports for the colo- 

 nies are generally rounded twigs, contains almost exclu- 

 sively variety c, and occasionally variety a, wherever 

 the surface is sufficiently extensive. Farther on, as the 

 stream broadens and the current is less swift, variety h 

 makes its appearance, and, is the preponderating form. 

 The other waters in which this species has been observed, 

 namely, Pennissewassee Pond, Norway, Me. ; Fresh Pond, 

 Cambridge, Mass. ; Schuylkill River at Philadelphia ; and 

 Gwinn's Falls, Baltimore, Md., are comparatively still, 

 and contained but one variety, variety b, the same that 

 occurs in the more sheltered spots of Young's Brook. 



The colonies of variety h rarely attain any considera- 

 ble size, while in variety c they sometimes form thick 

 clusters one or two inches in depth by three or four in 

 length. Besides these three varieties, there is another 

 probably belonging to this species, found in the outlet 

 of Great Pond. The colonies are small, consisting only 

 of a very few polypides. The brook formerly emptied 

 a large fresh-water pond ; but this has been drained, and 

 now at high tide the water becomes quite brackish, which 

 probably accounts for the diminutive size of the colonies. 



PLUMATELLID^. 



Coenoecium attached, branching. Lophopus with two 

 arms, stranding out like the arms of a horse-shoe. Stato- 

 blasts annulated and spiney, the spines passing through 

 the annulus. 



PLUM ATE LL A. 



Coenoecium, with shorter cells than in Fredericella, 

 covered by the thin ectocyst. Invagiuated fold broad, 



