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. 6, 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 Mcnocystidea being retained for the ordinary forms. As regards the size of the Gregarinoe, 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 (Toenia 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 movements can be effected, not, however, by pseudopodia. Haeckel regards the Gregarinoe, as Amoeboe 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.
Fig. 6. - Morphology and development of Gregarinida (after Stein and Lieberkuhn). a Stylorhynchus oligacanthus, a "dicystidean" Gregarine; b Gregarine of the earth-worm ("monocystidean"); c The same encysted; d Further stage of the same, with the contents divided into "pseudonavicellae;" e Free "pseudonavi-cellae;" f Amoebiform mass of protoplasm liberated from a pseudonavicella; g and h Active forms of f. All the figures are greatly enlarged.
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 invariable ; 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. 6, 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 "pseudonavicellae" (or "pseudonaviculae," fig. 6, d). The next step in the process consists in the liberation of the pseudonavicellae, which escape by the rupture of the enclosing cyst (fig. 6, 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 pseudona-vicella in this condition is very similar to an adult Amoeba (fig. 6, f, g, h). Finally, these amoebiform bodies are developed into adult Gregarinoe. It will be seen from the above that the formation of the pseudonavicellae out of the granular contents of the body, subsequent to the disappearance of the nucleus, presents a close analogy to the segmentation of the impregnated 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 Amoeba 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 "pseudo-filarige," 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.