422 RiSPOjiT OF THE State Geologist. 



space within this border is called the area^ and is frequently 

 entirely occupied by a chitinous membrane, in which is the cell- 

 mouth. In the fossil state the membrane has disappeared, the 

 elevated border alone remaining, the whole " area" appearing as 

 the mouth of the cell. The loss of all chitinous portions of the 

 cell and chitinous appendages will, of course, greatly change its 

 appearance, making it much more difficult to classify fossil than 

 recent forms The ectocyst is not, as has often been supposed, a 

 calcareous exudation from the surface of the animal, but is 

 deposited in a tegumentary membrane, forming not a mere cal- 

 careous crust, but an integral portion of the animal itself, which, 

 like the cartilage of higher animals, hardens by the deposition of 

 calcareous matter, but still is the seat of nutritive movement. 



Milne-Edward has made a number of experiments on the 

 cell walls of Esoharid^, and the results are here given in 

 translation. In the Ann. des Sciences J^at. ZooL, Yol. I, 

 pp. 25-31, he says: "If the stouy cells of the EscharidaB 

 were formed by the exudation of a calcareous matter which 

 molded itself on the surface of the secreting membrane, it is 

 evident that the first layer thus formed must be the external 

 one, and that the addition of new quantities of this earthy mat- 

 ter could only augment the thickness of the parietes of the cell 

 and modify the disposition of its interior cavity, without at all 

 changing the exterior configuration of the first formed layer ; 

 for here the solid cell completely envelops the animal and is not 

 overlapped by the secreting organ, as in the MoUusca gasteropoda, 

 whose shell changes its form with age, because the deposit of 

 new matter taking place on the border of the part already con- 

 solidated continually lengthens it and is molded on the soft 

 parts whose configuration is liable to change. 



" To throw some light on the mode of formation and on the 

 nature of the cells of the Eschares, it becomes, consequently, 

 interesting to examine these cells at different ages and to see if 

 their exterior form changed or remained always the same. This 

 study, indispensable for the anatomical and physiological history 

 of these little beings, may also lead to a knowledge useful to 

 zoology and geology, for the determination of the species, recent 

 and fossil, rests principally on the characters furnished by these 

 cells. And we are still ignorant whether or not they can be 

 modified in the progress of age. 



