24 LOWER JNVERTEBRATES. 



of a single cell frequently divided into two or more portions by transverse, internal 

 septa, forming the anterior and posterior sacs. In some species the anterior end, or 

 head, is furnished with a circle of reflexed hooklets, in others no such armature is found. 

 The membrane covering the body is transparent and structureless in appearance. 

 Although firm and elastic, it is very permeable to watery fluids. The contents of the 

 body are a nucleus embedded in the protoplasm and fatty granules. The latter are 

 usually so abundant as to give a milky appearance to the cell contents. The granules 

 seem to increase in size and abundance as the animal matures, since in young individ- 

 uals they are scarcely noticeable. 



The nucleus invariably present in all Gregarines is a spherical vesicle with a sharply- 

 defined membrane, situated near the middle of the cell in the Monocystidea and in the 

 forward part of the posterior sac of the Dycistidea. A nucleolus is usually found 

 within it. 



When about to multiply, the Gregarinje become surrounded by a transparent coat 

 or cyst, which may include either a single specimen or two together. After becoming 

 encysted a change takes ])lace in the enclosed Gregarines. If two of them are in 

 the cyst they become united, lose their identity and merge into a single mass. From 

 this mass nucleated cells soon develop, which gradually take the form of elongated 

 bodies, tapering at both ends, greatly resembling certain diatoms known as Naviculce, 

 whence they have taken their name, pseudo-uaviculae. They are also known as 

 psorosperms. Finally the membrane of the cyst is ruptured and the pseudo-naviculse 

 escape. 



The process above described is not the only one by which the spores are formed, 

 even in the same species. At least two others are known, one a process resembling the 

 segmentation of an egg, in which the entire mass is converted into very regular and 

 granular spheres of segmentation, whicli in turn becomes elongate and covered with a 

 firm investment, while their contents become more fluid ; another in which the con- 

 tents, instead of producing granular spheres, divides into two, four, or more parts, each 

 of which, by a process not understood, becomes covered with a layer of transparent or 

 slightly granular globules, and these parts are transformed into the spores. 



In some instances the cysts of Gregarines have been observed arranged in a linear 

 series in the walls of the rectum of certain animals, and it was for a long time a great 

 mystery how they could be thus regularly placed. Van Beneden observed in the rec- 

 tum of a lobster as many as seven cysts in a linear series. The Gregarine of the lobster, 

 Porospora gi(;antea, attains the extraordinary length of 16 mm. and .15 mm. in diame- 

 ter. During the proper season, spring and summer, it is very abundant in the intestine 

 of the lobster — at least in lobsters from certain localities — as many as twenty-five 

 being sometimes found in a single individual. At this time no cysts can be found, but 

 in the autumn the parasites seem to pass down into the rectum, where they become 

 encysted. The general process of encystment is somewhat as follows : — 



The contents of the cyst are always at first granular, forming a single sphere with- 

 out a nucleus. By division, two rounded masses next appear, and as the diameter of 

 the cyst increases, these separate, and a clear, colorless liquid surrounds them. The 

 wall of the original cyst then, at least in P. giguntea, becomes granular and disappears, 

 while the two globes become surrounded by firm membranes, and their contents may 

 again divide like the parent cyst. In other words, the cysts are capable of multiplica- 

 tion by division through successive generations. It is by the multiplication of the cysts 

 that their linear arrangement is brought about. Eventually the cysts cease to divide, 



