CHAPTER XIII.
PROTOZÓA (πρῶτος, FIRST, ξῶον, ANIMAL).
THE members of this subkingdom are the lowest in the scale of animal organization, their bodies consisting of a soft gelatinous and structureless mass, which has a remarkable tendency to form little cavities or vacuoles in its substance, and is called sar´code (σἀρξ, flesh). They exhibit no organs, unless the cilia and certain variable processes formed of the common substance of the body, and which form their agents of locomotion, be considered as such,—this substance exercising the combined functions of motion, sensation, and secretion, for which separate organs exist in the higher animals.
Rhizop´oda (ῥἱξα, root, ποῦς, foot).—The animals belonging to this class consist of the structureless colourless substance to which reference has been made as sarcode, and they exhibit no organs. The sarcodic body is slowly contractile, and portions of it can be protruded at will in the form of irregular root-like processes, acting both as legs for locomotion and as tentacles by which the animal grasps its prey, which is then forced into the substance of the body, where it becomes surrounded by the surface, and a cavity is formed, within which it is digested.
Amœ´ba dif´fluens ([Pl. XI.] fig. 10) is common in water in which portions of plants have been kept for some time. When first placed on the slide, the body appears as a minute, transparent, rounded mass of jelly; but if observed for some time, it will be seen slowly to protrude its root-like processes; and foreign bodies, as Diatomaceæ or other minute Algæ, will often be found imbedded in its substance.
Arcel´la vulgáris ([Pl. XI.] fig. 6) is found among Confervæ in ponds and ditch-water. It is contained in a hemispherical shell or carapace, from the round orifice of which the lobed processes are protruded. The shell is covered with minute pits.
Arcel´la aculeáta ([Pl. XI.] fig. 7) has the convex shell furnished with spines; fig. 8 represents the animal with its processes extended; while Arcella dentáta ([Pl. XI.] fig. 9) exhibits an angular or somewhat toothed membranous shell. Both the latter species are met with in the same localities as the first.
Actínophrys sol ([Pl. XI.] fig. 12) is a very beautiful and excessively delicate Rhizopod. The body is spherical, and covered with very delicate and slender cilia-like processes. Its movements are exceedingly slow, and can only be observed by prolonged watching. The body appears to be reticulated, from the presence of numerous vacuoles.
Two large groups of genera and species of Rhizopoda, the animal bodies possessing the above general characters, mostly with very slender processes, exist, in one of which (the Foraminifera) they are contained in calcareous shells, often of elegant forms; while in the other (the Polycystina) the shells are siliceous or composed of flint, both kinds of shells being perforated with holes. These shells, which occur in the fossil state in enormous numbers, sometimes forming mountain-masses, are extremely beautiful objects for the microscope.
Spon´giæ.—This class contains the Sponges, almost all of which are marine and foreign, and therefore not likely to come under observation in the perfect state. The substance commonly called sponge is the horny skeleton of the animal, consisting usually of rounded fibres ([Pl. XI.] fig. 13), irregularly netted and interlacing. The surface of a sponge exhibits minute pores and larger pouting orifices; the former of which admit currents of water, to be discharged at the latter, both being the mouths of continuous channels. The surfaces of the channels are lined with sarcodic matter, which takes the form of ciliated amœbiform bodies, by which the currents of liquid are produced.
The horny fibres of sponges are strengthened by little siliceous or flinty bodies of various forms ([Pl. XI.] fig. 13 a, b, c), which are imbedded in the substance of the fibres or attached to their surface, and form very curious microscopic objects. They are called spic´ula (spiculum, a dart), being often of a pointed form. In some sponges they are calcareous.