and a central" nucleus" is present, when the organism may be compared with a single "cell" in one of the higher animals.

In no Protozoön are any traces known of anything like the nervous and vascular arrangements which are found in animals of a higher grade. A nervous system is universally and entirely absent, and the sole circulatory apparatus consists in certain clear spaces called "contractile vesicles," which are found in some species, and which doubtfully perform the functions of a heart. A distinct alimentary aperture is present in the higher Protozoa, but in many there is none; and in all, the digestive apparatus is of the simplest character. Organs of generation, or at any rate differentiated portions of the body which act as these, are sometimes present; but in many cases true sexual reproduction has not hitherto been shown to exist.

The "sarcode," which forms such a distinctive feature in all the Protozoa, is a structureless albuminous substance, not possessing "permanent distinction or separation of parts," but nevertheless displaying all "the essential properties and characters of vitality," being capable of assimilation and excretion, of irritability and of the power of contraction, so as to produce movements, strictly analogous, in many cases, to the muscular movements of the higher animals. In some, too, the sarcode possesses the power of producing an external case or envelope, usually of carbonate of lime or flint, and often of a very complicated and mathematically regular structure.

The power of active locomotion is enjoyed by a great many of the Protozoa; but in some cases this is very limited, and in other cases the animal is permanently fixed in its adult condition. The apparatus of locomotion in the Protozoa is of a very varied nature. In many cases, especially in the higher forms, movements are effected by means of the little hair-like processes which are known as "cilia," and which have the power of lashing to and fro or vibrating with great rapidity. In other cases the cilia are accompanied or replaced by one or more long whip-like bristles, which act in the same fashion, and are known as "flagella." The most characteristic organs of locomotion amongst the lower Protozoa are known as "pseudopodia," and consist simply of prolongations of the sarcodic substance of the body, which can usually be emitted from the greater portion of the general surface of the body, and are capable of being again retracted, and of fusing completely with the body-substance.

2. Classification of the Protozoa.-The sub-kingdom Protozoa is divided into three classes-viz., the Gregarinida, the Rhizo

poda, and the Infusoria. In the Infusoria only is a mouth present, and hence these are sometimes spoken of as the "Stomatode" Protozoa, whilst the two former classes collectively constitute the "Astomata."

The following is a tabular view of the divisions of the Protozoa :

3. CLASS I. GREGARINIDE.-The Gregarinida may be defined as parasitic Protozoa, which are destitute of a mouth, and do not possess the power of emitting "pseudopodia." They constitute the lowest class of the Protozoa, and comprise certain microscopic animals which are parasitic in the alimentary canal of both Invertebrate and Vertebrate animals. They have, however, a special liking for the intestines of certain insects, being commonly found abundantly in the cockroach. As we shall see hereafter, in all probability a great deal of the degraded character of the Gregarinida is due to the fact that they are internal parasites, and are therefore not dependent upon their own exertions for food.

Nothing anatomically could be more simple than the structure of a Gregarina, since it is almost exactly that of a cell, such as the impregnated ovum (fig. 5, b). An adult Gregarina, in fact, may be said to be a single cell, consisting of an ill-defined membranous envelope filled with a more or less granular sarcode with fatty particles, and sometimes differentiated into a distinct contractile "cortical layer," which contains in its interior a vesicular nucleus, this in turn enclosing a solid particle, or nucleolus. In some the body exhibits an approach to a more complex structure by the presence of internal septa; but it is doubtful whether this appearance may not be due to the apposition and fusion of two separate individuals. A separate order,

however, has been founded upon individuals of this kind, under the name of Dicystidea; the name Monocystidea being retained for the ordinary forms. As regards the size of the Gregarine, they vary from about the size of the head of a small pin up to as much as half an inch in length, when they assume the aspect of small worms. The integument or cuticle with which the protoplasmic body is enclosed may be quite smooth or striated, or it may be furnished with bristles or spines, or even in some cases with cilia. Sometimes one end of the body is furnished with uncinate processes, very similar in appearance to the hooked "head" of the common tape-worm (Tania solium). Essentially, however, the structure of all appears to be the same. No differentiated organs of any kind beyond the nucleus and nucleolus exist, and both assimilation and excretion must be performed simply by the general surface of the body. The body is, nevertheless, contractile, and slow

Fig. 5.-Morphology and development of Gregarinida (after Stein and Lieberkühn). a Stylorhynchus oligacanthus, a "dicystidean" Gregarine; Gregarine of the earth-worm ("monocystidean"); The same encysted; d Further stage of the same, with the contents divided into "pseudonavicella;" e Free "pseudonavicellæ;"Amoebiform mass of protoplasm liberated from a pseudonavicella; g and A Active forms of f. All the figures are greatly enlarged.

movements can be effected, not, however, by pseudopodia. Hæckel regards the Gregarina as Amaba which have become degenerate by parasitism; but this opinion is rejected by Van Beneden, and their apparently unicellular structure would

rather lead us to place them in the neighbourhood of the Infusoria. The presence of muscular fibres in the cortical layer will also support the view that they should be associated with the Infusorian animalcules.

In spite of their exceedingly simple structure, the following very interesting reproductive phenomena have been observed sometimes in a single Gregarina without apparent cause, sometimes as the result of the apposition and coalescence of two individuals-the exact nature of the process being in either case obscure. In some species conjugation is in variable; in others it never occurs; and it may take place either by analogous or by opposite extremities. The Gregarina-or it may be two individuals which have come into contact and adhered together-assumes a globular form, becomes motionless, and develops round itself a structureless envelope or cyst, when it is said to be "encysted" (fig. 5, c). The central nucleus then disappears, apparently by dissolution, whereupon the granular contents of the cyst break up into a number of little rounded masses, which gradually elongate and become lanceolate, when they are termed "pseudonavicella" (or "pseudonaviculæ," fig. 5, d). The next step in the process consists in the liberation of the pseudonavicellæ, which escape by the rupture of the enclosing cyst (fig. 5, e). If they now find a congenial habitat, they give origin to little albuminous or sarcodic masses, which exhibit lively movements, and are endowed with the power of throwing out and retracting little processes of the body which closely resemble the "pseudopodia" of the Rhizopoda; so that the pseudonavicella in this condition is very similar to an adult Amaba (fig. 5, f, g, h). Finally, these amoebiform bodies are developed into adult Gregarina. It will be seen from the above that the formation of the pseudonavicellæ out of the granular contents of the body, subsequent to the disappearance of the nucleus, presents a close analogy to the segmentation of the impreg nated ovum which follows upon the dissolution of the germinal vesicle. In Gregarina gigantea of the Lobster the embryo is a little mass of sarcode, quite like an Amaba except that it wants a nucleus and contractile vesicle. It soon gives out two little contractile processes or arms, which become detached and move about like little worms, when they are termed "pseudofilariæ," from their resemblance to free Nematoids. After a period of activity, the pseudo-filarian becomes quiescent, shortens its dimensions, develops a nucleus and nucleolus, and becomes an adult Gregarina.

PSOROSPERMIÆ.-There occur as parasites on and within the bodies of fishes certain vesicular, usually caudate, bodies, termed Psorospermia, the exact nature of which is very problematical. According to Lieberkühn they occasionally give origin to amœbiform bodies, similar to those which are liberated from the pseudonavicellæ of Gregarinida. In this case they should probably be regarded as the embryonic forms of some Gregarina. By Balbiani, however, they are looked upon as properly belonging to the vegetable kingdom.

CHAPTER II.

RHIZOPODA.

GENERAL CHARACTERS OF THE RHIZOPODA.-The Rhizopoda may be defined as Protozoa which are destitute of a mouth, are simple or compound, and possess the power of emitting "pseudopodia." They are mostly small, but some of the composite forms, such as the sponges, may attain a very considerable size. Structurally, a typical Rhizopod-as an Amœba—is composed of almost structureless sarcode, without any organs appropriated to the function of digestion, and possessing the power of throwing out processes of its substance so as to constitute adventitious limbs. These are termed "pseudopodia," or false feet, and are usually protrusible at will from different parts of the body, into the substance of which they again melt when they are retracted. They are merely filaments of sarcode, sometimes very delicate and of considerable length, at other times more like finger-shaped processes; and they are identical with the little processes which can be thrown out by the white corpuscles of the blood and by pus-cells. Indeed, it has been remarked by Huxley that an Amaba is structurally "a mere colourless blood-corpuscle, leading an independent life."

The class Rhizopoda is divided into five orders-viz., the Monera, the Amabea, the Foraminifera, the Radiolaria, and the Spongida, of which the last is occasionally considered as a separate class.

ORDER I. MONERA. This name has been proposed by Hæckel for certain singular organisms which may provisionally be regarded as the lowest group of the Rhizopoda. They are very minute in size, and are distinguished by the fact that the body is composed of structureless sarcode, capable of emitting thread-like prolongations or pseudopodia, but destitute of either nucleus or contractile vesicle. The pseudopodia are mostly in the form of delicate filamentous processes of sarcode, which exhibit a circulation of minute molecules and granules in their interior and along their edges. Sometimes the pseudopodia may be simple, as in Protamaba (fig. 6, a), or they may be ramified and anastomosing, as in Protogenes. The form of the body, though very mutable, may be simple; or the organism may form a kind of colony of protoplasmic masses united by their interlacing pseudopodia (as in Myxo