516 NEW YORK STATE MUSEUM 



to a free or holoplanktonic mode of existence. This view is based on the 

 following facts. 



In the Axonolipa we find the rhabdosome in all cases where the 

 proximal parts have been observed, suspended either by means of an 

 extremely thin, distinctly flexuous nema [[).614, and pl.l2] or fastened 

 directly to a disk without the intercalation of a nema. The former mode 

 of suspension is observed among the lighter, thin branched forms and all 

 the younger individuals [pl.l2], while the unwieldy attachment of the 

 base of the rhabdosome to a disk occurs among the mature, heavy branched 

 forms of larger dimensions, notably of the genera Dichograptus and 

 Didymograptus [pl.l3]. This mode of attachment is apparently well 

 adapted to a sessile existence on the underside of floating seaweeds, which 

 naturally will be exposed more or less to the commotion of the surface 

 waters. The young lighter forms could favorably meet these conditions 

 by a suspension which allowed ready yielding to the movements of the 

 water, thus preventing their being torn from their bases. As the rhabdo- 

 somes grew heavier, a stage would be reached where the thin, flexuous 

 nema would no longer furnish a safe attachment, and a strengthening of 

 the same would lead to an inflexibility which would result in breaking 

 off the basal stem in case of more violent commotions of the water. Then 

 the nema was gradually discarded, and the rhabdosome began to hug the 

 supporting seaweed closely. This condition is specially well shown by 

 the horizontal Didymograpti, as D, patulus and extensus, which, 

 while rigidly horizontal, attain a great length. In them a nema is never 

 observed, while the central disk has been seen by the writer to be closely 

 attached to or to surround the apical portion of the sicula [pi. 13]. It is 

 further to be noted that in the latter forms the branches did not attempt to 

 assume a reclined direction, sought by the others ; and thus voluntarily fore- 

 went the advantage of giving the thecae an upward direction, which, with 

 the supposed close attachment to the underside of a seaweed, would have 

 been a distinct disadvantage. It is therefore to be assumed that they were 

 attached to the more solid portions, the stems or median parts of the leaflike 



