GREGARINIDA. 23 



cretaceous and other periods. In many cases the grains are found to be casts of Fora- 

 minifera. These constitute as mucli as ninety per cent, of some rocks. In the yellowisli 

 limestone of Alabama such casts occur in gieat jierfection, constituting about one-third 

 of the rock. To obtain them, the stone has only to be treated with acid, wlien the 

 greenish casts can readily be j)icked out from the insoluble residue, consisting of sand 

 and finely-divided mineral particles. 



Ehrenberg was the first to observe the replacement of organic forms by mineral 

 matter, and he inferred that green sand was always formed by such a substitution. 

 Such casts can be found in limestones from Mullica Hill, and near Mount Holly, N. J., 

 from Drayton Hill, near Charleston, S. C, from the cretaceous rocks of Western Texas, 

 and from other localities. 



At the i)resenl time precisely similar casts of Foraminifera are being formed at the 

 bottom of the sea. In the year 1853 Count Pourtales found, off the coast of Georgia, 

 in the Gulf Stream, at a depth of one hundred and fifty fathoms, a bottom deposit 

 consisting of shells of Glohigerina and black sand in about equal pro])ortions. Similar 

 deposits were found also in the Gulf of Slexico and in various parts of the Gulf Stream. 

 With them are also found the living Foraminifera, so that there can be no question as 

 to the continuance of the process now. 



RoMYN Hitchcock. 



Class III. — GEEGARINIDA. 



The Gregarinida are a peculiar kind of animal parasites which inhabit the intestinal 

 canals of earthworms, insects, Crustacea, etc., the simplicity of whose structure leads 

 most authors to class them among the Protozoa. Their distinct mem- 

 branous investment, however, entitles them to a higher rank than any 

 of the Protozoa already described, and, although with very little power 

 of movement, and possessing no means of searching for and collecting 

 food, they are still structurally higher than the Amceba and its allies, 

 for a differentiation of parts is certainly distinctly shown in the mem- 

 branous cell-covering. As parasites they do not require to move about 

 in search of food. They have no mouth, no organs of digestion. They 

 absorb their food through the membrane that covers the body ; hence, 

 although they are structurally above the Amoeba, they have almost lost 

 the Amoeba's power of voluntary movement. We may conceive of an 

 Amceba placed under conditions that would insure an abundant supply 

 of food without the necessity of searching for it, finally losing its power "^^'drma'-mmiln, 

 of movement and developing a distinct membranous investment from *" '"^^'^ ' 

 the ectosarc. We would then have a Gregarine. Van Beneden has regarded the 

 Gregarinida as Amoebae thus degenerated by parasitism. But there has been no degen- 

 eration of structure, only of habit ; it will be seen that Gregarines are all amoeboid in 

 one stage of their development, and that from this larval condition the more highly 

 differentiated adult is produced. 



The Gregarinida vary considerably in form and appearance, but in general terms 

 they may be described as more or less ovate or cylindrical in form, the body consisting 



