70 XINETRENTH REPORT. 



My own incomplete observations, coverina; some months with many 

 thriving cultures, give no direct evidence of hydrostatic activities, 

 although the bubbles themselves liave been numerous. I have never 

 found Arcellas in a location where they might not normally have arrived 

 by other means. 



My theory of the origin of the gas vacuoles is in line with Khainsky 

 elsewhere. The newly formed Arcella has a shell very soft and flexible 

 so tliat it yields to tlie contractions of tlie protoplasm. As it becomes 

 liardened, it no longer gives witli the same ease. The contracting pro- 

 toplasm pulls away from it, forming a slight vacuum, into wliich the 

 dissolved gas of the culture evolves. Under pressure again, this redis- 

 solves, thus accounting by a simple pliysical law for the irregularity of 

 the appearance and disaj^pearance of the vacuoles. Such a process would 

 alter the specific gravity of the organism, but whether it would be sufficient 

 to accomplisli flotation is not certain. 



THE SHELL. 



• 



General Features. On the basis of shell morphology, tlie genus Arcella 

 is customarily divided into half a dozen species. The type species, 

 A. vulgaris Ehgb. has been mentioned. A discoides Ehgb. is lower and 

 broader, generally shield-shape. Tn .1. mitrata Leidy the shell is mitri- 

 form, higher than wide, and widest at or near the middle, generally with 

 the sides of the dome divided into regular facets. A. dentata Ehgb. is 

 characterized by pronounced dentate processes around the base. 



In spite of these rather conspicuous differences, the forms intergrade 

 completely', so that it is impossible to be certain of the specific rating of 

 many individuals. The statement is made (Claparede and Lachmann 

 and others) that individuals of one form may produce others totally dis- 

 similar, so that the specific nomenclature is valid for convenience of 

 description only. Occasionally I have found an abnormal shell, but such 

 are quite exceptional. A noteworthy case was one with a double mouth 

 opening. Pseudopodia proceeded from one or the other without discrimi- 

 nation. Other abnormalities apjiarently have resulted from injury, with 

 incomplete regeneration of the part lost. 



Minute Anatomy. The details of the finer structure of the shell have 

 been the subject of mucli discussion. A regular cancellation of the sur- 

 face is apparent, even with moderate magnification, but the nature of 

 the construction is not so clear. The stock description has been that of 

 Leidy ('79), who states that the test "is composed of a more or less 

 translucent or trans})arent chitinoid membrane, with a minutely cancel- 

 lated hexagonal structure." From the text and figures, it is plain that 

 he considers the shell to be composed of closed hexagonal chambers like 



