LABYRINTHULIDEA.] PROTOZOA 843 out into very short fine filaments. Scattered in the protoplasm are a number of minute cylindrical crystals, of unascertained composi- ti in. Pelomyxa is of very largo size for a Protozoon, attaining a diameter of ^gth of an inch. It takes into its substance a quantity of foreign particles, both nutrient organic matter such as Ptotifera and Diatoms and sand particles. It occurs not uncommonly in old FIG. V. Lithamceba discus, Lank, (after Lankester, 34). A, quiescent ; B, throwing- out pseudopoclia. c.v., contractile vacuole, overlying which the vacuolated protoplasm is seen ; cone, concretions insoluble in dilute HC1 and dilute KHO, but soluble in strong HC1 ; n, nucleus. muddy ponds (such as duck-ponds), creeping upon the bottom, and has a white appearance to the naked eye. Lithamceba (Fig. V.) is distinguished by its large size, disk-like form, the disk-like shape of its pseudopodia, the presence of specific concretions, the vacuolation of its protoplasm, and the block-like form and peculiar tessellated appearance of its large nucleus, which has a very definite capsule. In Lithamceba it is easy to recognize a distinct pellicle or temporary cuticle which is formed upon the surface of the protoplasm, and bursts when a pseudopodium is formed. In fact it is the rupture of this pellicle which appears to be the proximate cause of the outflow of protoplasm as a pseudopodium. Probably a still more delicate pellicle always forms on the surface of naked protoplasm, and in the way just indicated determines by its rupture the form and the direction of the "flow" of protoplasm which is described as the "pro trusion" of a pseudopodium. The shells of Lobosa Testacea are not very complex. That of Arcella is remarkable for its hexagonal areolation, dark colour, and firm consistence ; it consists of a substance resembling chitin. That of Difllugia has a delicate membranous basis, but includes foreign particles, so as to resemble the built-up case of a Caddis worm. Arcella is remarkable among all Protozoa for its power of secret ing gas-vacuoles (observed also in an Amceba by Biitschli), which serve a hydrostatic function, causing the Arcella to float. The gas can be rapidly absorbed by the protoplasm, when the vacuole neces sarily disappears and the Arcella sinks. CLASS IV. LABYEINTHULIDEA. Characters. Gymnomyxa forming irregular heaps of ovoid nucleated cells, the protoplasm of which extends itself as a branching network or labyrinth of fine threads. The oval (spindle-shaped) corpuscles, consisting of dense protoplasm, and possessing each a well-marked nucleus (not observed in Chlamydomyxa), travel regu larly and continuously along the network of filaments. The oval corpuscles multiply by fission ; they also occasionally become encysted and divide into four spherical spores. The young forms developed from these spores presumably develop into colonies, but have not been observed. Genera. Two genera only of Labyrinthulidea are known : Labyrinthula, Cienkowski ; Chlamydomyxa, Archer. Cienkowski (35) discovered Labyrinthula on green Algte growing on wooden piles in the harbour of Odessa (marine). It has an orange colour and forms patches visible to the naked eye. Chlamy domyxa was discovered by Archer of Dublin (36) in the cells of Sphagnum and crawling on its surface ; hence it is a freshwater form. Unlike Labyrinthula, the latter forms a laminated shell of cellulose (Fig. VI. 2, c), in which it is frequently completely enclosed, and indeed has rarely been seen in the expanded labyrinthine condition. The laminated cellulose shells are very freely secreted, the organism frequently deserting one and forming another within or adherent to that previously occupied. The network of Chlamydomyxa appears to consist of hyaline threads of streaming protoplasm, whilst that of Labyrinthula has a more horny consistence, and is not regarded by Cienkowski as protoplasm. The spindle-shaped cells are much alike in form and size in the two genera ; but no nucleus was detected by Archer in those of Chlamydomyxa. The encysting of the spindle-cells and their fission into spores has been seen only in Labyrinthula. Chlamy domyxa is often of a brilliant green colour owing to the presence of chlorophyll corpuscles, and may exhibit a red or mottled red arid green appearance owing to the chemical change of the chlorophyll. It has been observed to take in solid nourishment, though Labyrin thula has not. The Labyrinthulidea present strong resemblances to the Myceto- zoa. The genus Uactylostelium (Sorophora) would come very close to Labyrinthula were the amoebic of its aggregation plasmodium FIG. VI. Labyrinthulidea. 1. A colony or "cell-heap" of Labyrinthula mtellina, Cienk., crawling upon an Alga. 2. A colony or "cell-heap" of Chlamydomyxa labyrinthitloid.es, Archer, with fully expanded network of threads on which the oat-shaped corpuscles (cells) are moving, o is an ingested food particle ; at c a portion of the general protoplasm has detached itself and become encysted. 3. A portion of the network of Labyrinthula vitellina, Cienk., more highly magnified, p, protoplasmic mass apparently produced by fusion of several filaments ; p , fusion of several cells which have lost their definite spindle-shaped contour ; s, corpuscles which have become spherical and are no longer moving (perhaps about to be encysted). 4. A single spindle cell and threads of Laby rinthula macrocystis, Cienk. n, nucleus. 5. A group of encysted cells of L. macrocystis, embedded in a tough secretion. 6, 7. Encysted cells of L. macrocystis, with enclosed protoplasm divided into four spores. 8. 9. Transverse division of a non-encysted spindle-cell of L. macrocystis. set upon a network of threads. Such a network, whether in the condition of soft protoplasm or hardened and horny, is represented in the higher Mycetozoa by the capillitium of the sporoeysts. The most important difference between Archer s Chlamydomyxa
and Cieiikowski s Labyrinthula is that in the former the threads