OBSERVATIONS ON POLYZOA. 



223 



PLTJMATELLA ARETHUSA Hyatt. 



The coenoecia have the usual range of variation. Va- 

 riety a has distinct cells and radiating branches. The 

 ectocyst may be either brown or colorless, and the cells 

 vary greatly in size (PI. 8, figs. 1, 2). 



Variety b has the branches and cells adherent, forming 

 thick encrustations upon the surfaces of branches or logs ; 

 the cells, however, appear to be distinct, the lower por- 

 tion not being obliterated to so great an extent as in 

 variety c of P. vesicularis and variety b of P. diffusa. 

 Variety a has generally a brownish ectocyst, but the 

 younger colonies or branches are colorless. Orifices are 

 channelled in the brown colonies, and entire in the color- 

 less ones. The posterior retentors consist of from 7 to 

 13 rows, and the anterior retentors have about 10 bands. 

 Tentacles from 40 to 60 in number. Statoblasts vary 

 from .199 m. m. in breadth by .266 m. m. in length, to 

 .266 in breadth by .399 m. m. in length. In equal num- 

 bers the statoblasts measure 6 by 8, 6 by 9, 6 by 10, 7 

 by 9 to 7 by 11, 8 by 11 to 8 by 12. 



There is, therefore, as in the preceding species, a 

 gradual increase in the breadth, and also a gradual 

 increase in length among the flill-grown statoblasts. 



8x12 



7x9 



6x8 



FIG. 15, three varieties of the statoblast ; a, a, back and front view of one speci- 

 men; b, b, back and front view of one specimen; c, c, back and front view of one 

 specimen. In this specimen the annulus covers the whole side of the statoblasts 

 on one side. 



Localities. Pennissewassee Pond at Norway, Gor- 

 ham, Great Falls, the outlet of Great Pond on Cape 

 Elizabeth, Presumpscot River, all in Maine ; Fresh Pond 

 and Mystic Pond at Cambridge, and Green River at 

 Greenfield, all in Massachusetts. 



