1829. Sub-order _Myxogastres_ Fries, _Syst. Myc._, III., p. 67.

1833. Sub-order _Myxomycetes_ Link, _Handb. der Gew._, 3, p. 405.

1833. Sub-order _Myxomycetes_ Wallroth, _Fl. Crypt._, II., p. 333, in part.

1858. Cla.s.s _Mycetozoa_ DeBary, _Bot. Zeitung_, 1858, pp. 357-365, in part.

1889. Cla.s.s _Myxogastres_ Schroeter, _Engl. u. Prantl_, Nat. Pflanz., I., i., p. 16.

1892. Cla.s.s _Myxogastres_ (Fries) Ma.s.see, _Monograph_, p. 28.

1894. Cla.s.s _Mycetozoa_ Lister, _Mycetozoa_, p. 21, in part.

Except as just described, the slime-moulds present abundant, minute, unicellular spores, enclosed in sporangia more or less perfectly defined, and attended by peculiar thread-like structures, free or variously attached and conjoined, the so-called _capillitium_.

So far as known, the spores on germination give rise to zoospores, at first amoeboid, later ciliate, again amoeboid, conjugating in pairs, then, in some cases, at least, coalescing and dividing indefinitely to form the plasmodial or vegetative phase.[17]

=Key to the Orders of the Myxogastres=

Spore-ma.s.s black or violaceous, rarely ferruginous Series A

Spore-ma.s.s never black; usually some shade of brown or yellow, rarely purplish or rosy, etc. Series B

SERIES A

1. Capillitium present, delicate, thread-like; sporangia calcareous more or less throughout I. PHYSARALES

2. Capillitium present, thread-like, arising usually as anastomosing branches from a well-developed columella, which in a single genus contains lime; sporangia otherwise non-calcareous II. STEMONITALES

SERIES B

3. Capillitium none, or very imperfectly developed; spores of some shade of brown, rarely purplish III. CRIBRARIALES

4. Capillitium the inwardly produced irregular extremities of plates or tubules, which by their interweaving outwardly make up the aethalial wall; spores pale, ashen IV. LYCOGALALES

5. Capillitium made up of more or less distinctly sculptured threads, parietal or free, simple, branched, or reticulate; spores commonly yellow V. TRICHIALES

This sequence is meant to convey the idea that the presence of lime is indicative of differentiation less complete. That the plasmodium should at the outset eliminate, by refusing the unnecessary lime, is indicative of higher rank than that the lime should be carried until the last and then be crystallized out, or excreted by simple desiccation. The circ.u.mstance that the excreted lime may sometimes serve a protective purpose in the fruit, does not vitiate the general principle. In Series B the differentiation reaches a climax in the sculptured capillitium of the trichias.

ORDER I

=PHYSARALES=

Spores violaceous-black. The capillitium usually delicate and thread-like; peridium and capillitium, one or other or both, more or less extensively surcharged with lime. Peridium simple or double.

Fructification various.

This order is recognizable by several characteristics, but is especially marked by the peculiar calcareous deposits which affect the capillitium or peridium, now one, now the other, more often both.

As here defined, the order Physarales includes two distinct families; of the one _Physarum_, of the other _Didymium_, is type.

=Key to the Families of the Order Physarales=

_A._ Fructification often calcareous throughout; capillitium intricate _Physaraceae_

_B._ Calcareous deposits, when present, affecting the peridium only, or sometimes the stipe, in the typical genus plainly crystalline; capillitium simple _Didymiaceae_

A. PHYSARACEae

=Key to the Genera of the Physaraceae=

_A._ Fructification aethalioid 1. _Fuligo_

_B._ Fructification plasmodiocarpous or of distinct sporangia.

_a._ Peridium evidently calcareous.

i. Capillitium calcareous throughout 2. _Badhamia_

ii. Capillitium largely hyaline.

O Sporangia globose, etc.; dehiscence irregular 3. _Physarum_

OO Sporangia vasiform or more or less tubular

+ Dehiscence by a lid or more or less circ.u.mscissile 4. _Craterium_

++ Dehiscence irregular, peridium introverted 5. _Physarella_

_b._ Peridium apparently limeless, at least outside.

i. Plasmodiocarpous 6. _Cienkowskia_

ii. Sporangia distinct 7. _Leocarpus_

C. Extra-limital.

_a._ Sporangia stipitate, saucer-shaped, following No. 3. _Trichamphora_

_b._ Sporangia elongate allantoid, etc., following No. 1. _Erionema_

=1. Fuligo= (_Haller_) _Pers._

1753. _Mucor_ Linn., _Sp. Pl._ II., No. 1656 (?).

1768. _Fuligo_ Haller, _Hist. Helv._, Nos. 1233-1235, in part.

1801. _Fuligo_ Haller, _Pers. Syn._, p. 159.

1809. _aethalium_ Link, _Diss._, I, p. 42.

1829. _aethalium_ Fries, _Sym. Myc._, III., p. 92.

Sporangia undefined, obscurely woven in and out among each other forming usually a cushion-shaped aethalioid ma.s.s. The outer layer sterile, often calcareous, forming a fragile crust, more or less defined. The middle layer sporiferous with calcigerous capillitium. The lowest layer a membranous hypothallus.

The ident.i.ty of this genus seems to have been recognized first by Haller, _op. cit._, but by Persoon more closely defined and ill.u.s.trated.