24 GENERAL HISTORY OF THE INFUSORIA. 



SUBFAMILY PEDIASTEE^. 



(Plate I. 37 to 69. Plate II. 19, 36, 37.) 



This includes the genera MicrasteHas and Arthrodesmus of Ehrenberg, the 

 Pediastrum and Scenedesmus of Ealfs, Kiitzing, and others ; and, in addition 

 to these two, to foUow NageU's classification, Soi^astrum, Coelastrmn, and 

 probably also Splicerodesmus. 



At the time Mr. Ealfs wrote, much nncertainty prevailed respecting what 

 shonld be considered characteristics of species, and what were the modes of 

 propagation ; and it is much to be regretted that, although some of the diffi- 

 culties and doubts are removed, oiu- knowledge of these microscopic Algae is 

 far from complete. 



Ehrenberg, in harmony mth the general views of organization he had 

 adopted, placed Mm^asterias and Arthrodesmus among the Desmidieae, in the 

 class of Polygastric Infusoria, and described the existence in them of ova, 

 stomach-vesicles, and seminal glands. Yet he was unable to point out one 

 single feature reaUy indicative of their animal natm-e, even locomotion being 

 unrecognized. Indeed, among those who might be inclined to foUow the 

 distinguished Berlin naturalist in attributing an animal nature to most of his 

 Polygastria, the generality would hesitate, in face of the many intimate ho- 

 mologies, structui-al and physiological, between the Pediastreae and admitted 

 AlgaB, to predicate it of that group of organisms. 



EiGTJRE, Composition, and Contents of Cells. — The individual cells among 

 the Pediastreae do not exist isolated and independent, but are united together 

 in a frond, in determinate number and in a definite arrangement for each 

 genus. In all the species they agree in having a membranous wall like the 

 Desmidieae and PalmeUeae. We confine ourselves, it should be imderstood, 

 in noting the figure of the cells, to the mature phase or stage, which, although 

 but one of several known phases, is that most marked, best understood, and 

 most perfect. 



The cells of Scenedesmus {Arthrodesmus, Ehr.) (I. 37 to 43) are entire, oval, 

 oblong, or fusiform, with their ends either rounded or pointed. Theii' length 

 is from two to four times theii' width or thickness, and they are spherical on 

 a transverse section. They exliibit no constriction or sutui^e at the middle, 

 neither in their waU nor in theii' endochrome, and in these particulars con- 

 sequently differ fr'om the cells of true Desmidieae. The membrane is fr^e- 

 quently drawn out in the form of straight or ciu'ved spines ; this happens 

 usually only with the ceU at each end of the chain (I. 40, 41) ; but in a few 

 cases, other cells nearest to the outer ones become also armed with spines 

 (I. 42). When this extension to the other cells occui^s, Nageli remarks that 

 the spines do not appear on both the superior and inferior extremities of each 

 cell, but only on the upper of the one, two, or three next within the one 

 terminal cell, and on the lower extremity of the same number within the 

 other terminal cell (I. 42). It is rare that the central cells of the chain are 

 armed, and even when this occurs it is only with short spines. In addition 

 to a spine, or, as may happen, a pair of spines from the upper and lower ex- 

 tremity, the end cells at times have a third spine standing at right angles 

 from their sides (I. 41). 



The cells of Pediastrum are considerably compressed, so that when aggre- 

 gated they form a flattened tabular stmctui-e (I. 44 to 48, 59 to 69). In 

 figui^e, as seen fr'om above, they vary according as they occupy the margin of 

 the collection, i. e. are peripheral, or are central. The latter are polygonal, 

 frequently hexagonal, and no doubt owe this shape to mutual lateral pressure 

 during growth : the marginal cells have fewer sides, and are frequently irre- 



