Infusoria

Infusoria, a class of Protozoa (q.v.) which includes the most specialised members of this phylum. The class is characterised by having a stable outline, as they do not emit pseudopodia (of. Amcrba), but have a more or less complete investment of the small vibratile processes known as cilia. They usually have a mouth (cytostome) and an anus (cytopyge). The body is often divided into two layers - the exoplasm (or cortex) and endoplasm (or medulla). A contractile vacuole (q.v.) is nearly always present. The nucleus is either single or multiple. They are mostly microscopic in size, but the compound forms may be fairly large. They are nearly all free-swimming, and live in either salt or fresh water; a few live in a small gelatinous tube or lorica, while others are attached by small pseudopodial processes (e.g. Stentor). The only skeletal structures known are that the lorica of some species contains particles of silica (as in Cordinella), while in a few it is more specialised, consisting of a siliceous tube perforated by minute pores, which give it a radiolarian (q.v.) aspect. There are four orders of Infusoria, mainly characterised by the disposition of the cilia. They are - (1) Holotricha, in which the cilia are of the same form and occur in parallel lines; (2) Ileterotricha, in which there is an investment of fine cilia, while there is also one band of larger processes known as "membranellas," which are formed of many fused cilia; (3) Hypotricha, in which the cilia occur only on the ventral side; and (4) Peritricha, in which there is one band of membranellse, but (with the exception of Triehodinopsis) there are no cilia. The Infusoria occur in great abundance in most rivers, ponds, and the sea, but they are only doubtfully known in a fossil condition. It should be remembered that the term Infusoria is often used in a more general sense to include also all the Mastigophora (q.v.).