The assemblage of forms, for the most part inhabitants of the fresh-waters, gathered together under the designation Proteomyxa (Ray Lankester), are characterised mainly by the negative feature that they cannot be assigned with certainty to any of the foregoing classes; nor is it possible to frame any satisfactory and common definition of them as a whole. The discovery of new forms, and a .better acquaintance with some of those already known, will in time lead, without doubt, to their dispersal.

A certain number of Proteomyxans have a Heliozoon-like aspect, which they may exchange for an irregular one; they may be colonial or capable of indefinite growth. Others have been grouped by Zopf as Monadineae. The typical feature of these forms is that they have two kinds of cysts, one (zoocyst) within which the organism breaks up into spores, the other (sporocyst) within which it contracts, encysts perhaps again, but eventually, assuming a spherical or oval shape and acquiring a membrane, passes into the state of a resting spore or chlamydospore, usually single. The spores originating from the zoocysts are either flagellulae s. zoospores, or amoe-bulae; hence a subdivision of the Monadineae into the M. zoosporeae and the M. azoosporeae. But in the M. zoosporeae, with the exception of Colpodella, the zoospore passes into an amoebula which grows in size and finally encysts. Fusion of Amoebae into plasmodia is known to occur in some instances; and, partly on the strength of this fact, and partly on account of the character of the spores (flagellulae and amoebulae), the formation of sporocysts or free chlamydospores, Zopf relegates the Monadineae to the Mycetozoa as 'lower Mycetozoa.' But they are all distinctly animal in their nutrition, with two exceptions, Haplococcus which lives between the muscle-fibres of swine, etc, and Bursulla which flourishes in horse-dung. Another difference, as compared with the Mycetozoa, is the fact, pointed out by Zopf himself, that the higher Mycetozoa have nothing to correspond to the zoocyst phase.

As will be seen from the account given below, our knowledge of some of the forms is very defective, and others do not follow completely the typical life-history.

1Ray Lankester is of an opinion that the spindles in the two genera above-described are really nuclei. Q. J. M. xix. 1879, P. 481.

There remain some Amoeboid organisms about which little is known, differentiated from one another by shape, character of the pseudopodia, mode of locomotion and of occurrence. Many of them, like some of the forms of Heliozoon-aspect, are non-nucleate, or are said to be non-nucleate. The latter were grouped together by Haeckel as Monera, but it is doubtful how far the distinction, even if found to*hold good, justifies such an association 1.

The following epitome of the Proteomyxa is given principally for the use of persons who may take an interest in these simple organisms.

Heliozoon-like genera; Nuclearia, Myxastrum, Archerina, Monobia, Myxodyc-tium. The first-named is freshwater, globular when in a state of rest, elongate and somewhat lobed when in motion. Its pseudopodia are long, pointed and branched at an acute angle, radiating or confined to a portion of the body when in motion. It is frequently surrounded by a hyaline gelatinous layer beset with minute rods. It has one or many nuclei, and a number of slowly contractile vacuoles. It lives on A/ga-cells, the contents of which it sucks out. A double cyst has been observed in one of the two species. Myxastrum is marine; it is globular with radiant pseudopodia, long, pointed, in one species sometimes branched at an acute angle, and anastomosing. M. Liguricum is multinucleate, M. radians is said by Haeckel to be non-nucleate. The latter forms a structureless membraneless cyst, within which the protoplasm segments into a number of radially arranged oval spores. Each spore acquires a siliceous investment, and gives exit to a minute Myxastrum. Archerina is freshwater and non-nucleate. It is Actinophrys-like, and 1/2000 of an inch in diameter. It has motionless radiant pseudopodia, one or more non-contractile vacuoles, and a bifid or double chlorophyl body.

The chlorophyl body divides into four and does so repeatedly; at the same time the protoplasm increases in amount, and becomes irregular in shape; it gives off lobose processes while the radiant pseudopodia are confined to a few spots. Ingestion of Bacteria was observed. The colonies thus produced appear to grow till food is scarce, and then to break up. Each chlorophyl body carries away with it a portion of protoplasm. Large encysted individuals occur; the cyst membrane is resistent. The chlorophyl may be disseminated through the encysted mass. The two remaining Heliozoon-like genera are also both non-nucleate, but are characterised by the fact that they form colonies. Monobia confluens lives in freshwater, and perhaps damp earth; its pseudopodia are radiant with minute varicosities. It multiplies by fission, and the two individuals thus formed may remain united by a bridge of protoplasm. Union may take place also by the pseudopodia. Myxodyctium sociale is marine; the colony is formed by the union of the branching and anastomosing pseudopodia.

1The evidence for the complete absence of a nucleus rests in some instances on the authority of observers, working with full command of the most modern histological methods, e. g. Ray Lankester on Archerina, Gruber on Prolamoeba vorax. See also a paper by Brandt, 'Microchemische Untersuchungen,' S.B. Physiol. Ges. Berlin, 1879. Gruber has pointed out that non-nucleate parts of individuals may grow in size, heal a wound, carry on to complete development an organ which has begun to develope, but they are incapable of starting the development of an organ and of reproduction; see Biol. Centralblatt. iii. p. 580, and A. N. H. (5), xvii. p. 482. But non-nucleate organisms such as Archerina or Monobia are capable of reproduction. The cases are therefore not parallel.