Mineral Deposit in the Rhizopods and Sponges. 79 



&c., in the Number of the ' Aunals' for June last (PI. X. fig. 13) 

 and in those forms of Arcella in which a number of spine-like 

 processes are formed, and the original chitinous test becomes 

 covered with sandy particles, it is probably through apertures 

 in these that the sarcode escapes. 



Lastly, it is a most interesting fact, that, if we turn to the Pro- 

 tophytes, as, for- example, the Diatoms and Desmidians, a layer 

 of protoplasm, as is well known, envelopes the harder portions 

 exteriorly. This layer is homologous with the chitosarc of the 

 Rhizopods. 



Now one of the strongest corroborations of the view here ad- 

 vanced is to be found in the fact that in an abnormal condition 

 of the oldest Globigerine shells, in which the larger foramina 

 become almost wholly obliterated by calcareous deposit (and 

 it would appear that elective affinity exercises greater power 

 than the inherent secretory faculty of the sarcode), a secondary 

 free layer of shell-substance becomes deposited within the 

 primary layer ; and from this the delicate calcareous spines pre- 

 sent in some of the heaviest of the free-floating surface Globi- 

 gerinse of tropical seas seem to be projected. At first sight 

 these spines look like pseudopodia ; they are undoubtedly cal- 

 careous, however, and, as before stated, never tubular. 



In the diagram a second layer is represented as having been 

 already added on the external surface of the primary chamber, 

 p c, — this layer extending, however, only over the external 

 area of the primordial chamber, and not over that portion of it 

 which is covered by the second chamber, s c. The latter is repre- 

 sented as seen prior to the deposition of this outer or secondary 

 layer — the spreading out of the stolons, so as to form the chi- 

 tosarc, having yet to take place before any external addition to the 

 shell can be brought about. Thus it has been shown that the 

 deposit of mineral matter, of which the Foraminiferous shell is 

 composed, takes place only in one direction at a time, and not 

 within a cavity, but either upon or within a surface of sarcode. 



The same remark applies to the spinous projections which 

 are occasionally present. These may be traversed by a canal- 

 system, but they never exhibit a tubule originally occupied by a 

 stolon from which they have been partly secreted. On the con- 

 trary, they are built up of consecutive additions of calcareous 

 matter, laid on, as it were, at right angles to their axes, and 

 extending in one direction only, that is to say, from the axis of 

 the chamber outwards ; whilst the secreting surface constitutes a 

 progressive mould into which the mineral matter is poured out, 

 the base of the secreting cavity being, at the commencement of 

 the operation, closed by the calcareous area upon which the 

 spinous process is projected. Here it is evident that the in- 



