86o THE ANIMAL KINGDOM. 



in Monocystis, &c. ; or the anterior extremity of one individual with the 

 posterior of another (opposition), as in Polycystids, leading occasionally to 

 the formation of strings of individuals. The union is more or less firm l . 

 Solitary individuals may encyst, or conjugated individuals may become in- 

 closed in a common cyst. They may, as is probably generally the case, be 

 separated by a septum, or they may rotate over one another and perhaps 

 in some instances fuse. Encystation is always accompanied by the 

 assumption of a globular shape. The cyst is said in Monocystis to be 

 formed by the cuticle and sarcocyte ; in the majority it is certainly a new 

 structure. It may be single or double, and the outer layer sometimes 

 thick and concentrically striate. A delicate inner membrane is present 

 when there are special sporoducts (Gamocystis, Clepsidrina). The outer 

 membrane is ornamented with tubercles, &c., in Stylorhynchus and its 

 allies. The size of the cyst varies ; it is resistent to the action of water. 

 It undergoes fission in Porospora gigantea ; the two first formed halves 

 divide a second time, and the old cyst-wall degenerates into a transparent 

 mass imbedding the cysts secreted by the new parts. 



The contents of the cyst proceed to develope either while it is in situ, 

 or after it is expelled in the faeces, if the Gregarine inhabits the digestive 

 tract. The nucleus becomes invisible ; many superficial nuclei, doubtless 

 derived, as in Coccidiidae, by fission of the primitive nucleus, have been 

 observed in Clepsidrina Blattarum. The protoplasm either segments 

 entirely into a number of nucleated sporoblasts, or only the superficial zone 

 does so. In the Monocystids of the Earthworm, the sporoblasts are said 

 to be formed centrally, to be extruded successively, and arranged in con- 

 centric zones, until the whole protoplasm is used up. They are formed 

 as buds projecting from the surface in Clepsidrina and Stylorhynchus, in 

 the last-named motile. The protoplasm not used up is usually resolved 

 into a fluid ; in Stylorhynchus it collects into a central spherical mass, 

 the pseudocyst. The sporoblast assumes by degrees its definitive shape, 

 elliptical and pointed at the ends, which in Syncystis have four bristles, 

 cylindrical with conical truncate ends, as in Clepsidrina, or purse- shaped, 

 as in Stylorhynchus and its allies. In Urospora it has a tail 2 . It acquires 

 an envelope, single or double, as in Clepsidrina, bivalved in Adelea, porous 

 in Porospora, where it is readily resolved into minute rods ; brown or 

 black in Stylorhynchus and its congeners. Spores belonging to the same 

 species may be large in one cyst, small in another (macro-, micro-spores). 

 Except in Porospora gigantea the contents are resolved into falciform 



1 The genus Didymophydes of Stein was founded on two conjugated Sporonts. Frenzel has 

 established a genus Aggregata for a Polycystid inhabiting the intestine of Cancer Maenas, which 

 conjugates in strings of individuals. 



2 The elliptical spore pointed at both ends is sometimes termed ' pseudo-navicella.' The term 

 ' psorosperm ' has been applied to the Coccidiidae, Gregarine spores, Myxosporidian spores, and to the 

 Sarcosporidia. 



