At the point where the body becomes actually intracellular, it is constricted, and this constriction marks off the epimerite (internally) from the middle portion (between this point and the septum), which is the protomerite. Further growth is restricted, practically, to the extracellular regions, and the epimerite often comes to appear ultimately as a small appendage at the anterior end of the protomerite. A Gregarine at this stage is known as a cephalont. Later on, the parasite breaks loose from the host-cell and becomes free in the lumen, the separation taking place at the constriction between the protomerite and the epimerite; the latter is left behind in the remains of the host-cell, the former becomes the anterior part of the free trophozoite.
In other Gregarines, however, those, namely, which pass inwards, ultimately becoming “coelomic,” as well as those which become entirely intracellular, no epimerite is ever developed, and, further, the body remains single or unseptate. These forms, which include, for instance, Monocystis (fig. 4), Lankesteria, Diplocystis, are distinguished, as Acephalina or Aseptata (Haplocyta, Monocystida), according to which character is referred to, from the others, termed Cephalina or Septata (Polycystida).
The two sets of terms are not, however, completely identical or interchangeable, for there are a few forms which possess an epimerite, but which lack the division into protomerite and deutomerite, and are hence known as Pseudomonocystida; this condition may be primitive (Doliocystis) or (possibly) secondary, the partition having in course of time disappeared. Again, Stenophora is a septate form which has become, secondarily, completely intracellular during the young stages, and, doubtless correlated with this, shows no sign of an epimerite.
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| From Wasielewski, after Léger. | |
| Fig. 7.—Forms of Epimerites. | |
1, Gregarina longa. 2, Sycia inopinata. 3, Pileocephalus heerii. 4, Stylorhynchus longicollis. 5, Beloides firmus. | 6, Cometoides crinitus. 7, Geneiorhynchus monnieri. 8, Echinomera hispida. 9, Pterocephalus nobilis. |
With regard to the epimerites themselves, they are of all variety of form and shape and need not be described in detail (fig. 7). In one or two cases, however, another variety of attaching organella is met with. Thus in Pterocephalus, only the rostrum of the sporozoite penetrates into the host-cell, and no epimerite is formed. Instead, a number of fine root-like processes are developed from near the anterior end, which pass in between the host-cells (fig. 5) and thus anchor the parasite firmly. Similarly, in the curious Schizogregarinae, the anterior end of the (unseptate) body forms a number of stiff, irregular processes, which perform the same function (fig. 8). It is to be noted that these processes are non-motile, and not in any way comparable to pseudopodia, to which they were formerly likened.
A very interesting and remarkable morphological peculiarity has been recently described by Léger (18) in the case of a new Gregarine, Taeniocystis. In this form the body is elongated and metamerically segmented, recalling that of a segmented worm, the adult trophozoites possessing numerous partitions or segments (each corresponding to the septum between the proto- and deuto-merite in an ordinary Polycystid), which divide up the cytoplasm into roughly equal compartments. Léger thinks only the deutomerite becomes thus segmented, the protomerite remaining small and undivided. The nucleus remains single, so that there is no question as to the unicellular or individual nature of the entire animal.
| After Léger and Hagenmüller, from Lankester’s Treatise on Zoology. |
| Fig. 8.—Three Individuals (G) of Ophryocystis schneideri, attached to wall of Malpighian tubule of Blaps sp. p, Syncytial protoplasm of the tubule; c, Cilia lining the lumen. |
The general cytoplasm usually consists of distinct ectoplasm and endoplasm, and is limited by a membrane or cuticle (epicyte), secreted by the former. The cuticle varies considerably in thickness, being well developed in active, intestinal Minute structure. forms, but very thin and delicate in non-motile coelomic forms (e.g. Diplodina). In the former case it may show longitudinal striations. The cuticle also forms the hooks or spines of many epimerites. The ectoplasm usually shows (fig. 9A) a differentiation into two layers, an outer, firmer layer, clear and hyaline, the sarcocyte, and an inner layer, the myocyte, which is formed of a network of muscle-fibrillae (mainly longitudinal and transverse, fig. 9B). The sarcocyte alone constitutes the septum, traversing the endoplasm, in septate Gregarines. The myonemes are undoubtedly the agents responsible for the active “gregarinoid” movements (of flexion and contraction) to be observed in many forms. The peculiar gliding movements were formerly thought to be produced by the extrusion of a gelatinous thread posteriorly, but Crawley (8) has recently ascribed them to a complicated succession of wave-like contractions of the myocyte layer. This view is supported by the fact that certain coelomic forms, like Diplodina and others, which either lack muscle-fibrils or else show no ectoplasmic differentiation at all, are non-motile. The endoplasm, or nutritive plasm, consists of a semi-fluid matrix in which are embedded vast numbers of grains and spherules of various kinds and of all sizes, representing an accumulation of food-material which is being stored up prior to reproduction. The largest and most abundant grains are of a substance termed para-glycogen, a carbohydrate; in addition, flattened lenticular platelets, of an albuminoid character, and highly-refringent granules often occur.
| After Schewiakoff, from Lankester’s Treatise on Zoology. | |
| Fig. 9a.—Longitudinal section of a Gregarine in the region of the septum between protomerite and deutomerite. Pr, Protomerite. De, Deutomerite. s, Septum. en, Endoplasm. sc, Sarcocyte. c, Cuticle. m, f, Myocyte fibrils (cut across). g, Gelatinous layer. | Fig. 9b.—Gregarina munieri, showing the network of myocyte fibrillae. |
The nucleus is always lodged in the endoplasm, and, in the septate forms, in the deutomeritic half of the body. It is normally spherical and always limited by a distinct nuclear membrane, which itself often contains chromatin. The most characteristic feature of the nucleus is the deeply-staining, more or less vacuolated spherical karyosome (consisting of chromatin intimately bound up with a plastinoid basis) which is invariably present. In one or two instances (e.g. Diplocystis schneideri) the nucleus has more than one karyosome. All the chromatin of the nucleus is not, however, confined to the karyosome, some being in the form of grains in the nuclear sap; and in some cases at any rate (e.g. Diplodina, Lankesteria) there is a well-marked nuclear reticulum which is impregnated with granules and dots of chromatin.