Dr. WallicHj on the Polycystina. 
81 
inch; destitute of anything like a cell-wall; but shortly- 
after projection from the parent body, exhibiting what might 
easily, under casual inspection, be mistaken for a nucleus. 
This is, in reality, the siliceous rudiment of the skeleton of 
the new individual. As correctly shown by Miiller (who, by 
the way, failed to detect the derivation or future progress of 
the yellow bodies the sarcob lasts are to be seen resting, 
in more or less of a layer, immediately within the siliceous 
framework. Subsequently, however, they are projected 
through the foramina, and gradually thrown off altogether. 
Occasionally, during calms within the tropics, the sarco- 
blasts of the Polycystina and other oceanic Rhizopods may be 
taken in immense numbers, although, owing to their extreme 
minuteness, they are easily overlooked. The profusion, how- 
ever, in which they occur, in every stage of growth, affords 
us the means of tracing their history in all its consecutive 
phases ; and it is highly desirable that they should be care- 
fully collected and studied by all who enjoy opportunities of 
obtaining them in their normal condition. I may add that 
the fossil Barbadoes earth generally contains numbers of the 
denuded omphalostypes, even in the earliest stages of their 
history."^ 
Without entering at present into the questions whether 
the sarcoblast ought or not to be regarded as a true ovum ; or 
the siliceous deposit on the ske]e4;on of the Polycystina should 
be regarded as a true secretion or merely as an exudation, 
or, lastly, as the eombined production of vital and chemical 
forces, it is certain that the development of the sarcoblast 
invariably precedes the first appearance of the embryonic ske- 
leton; and we are hence warranted in taking for granted that 
its deposition is not independent of the sarcodic body, as might 
be inferred were the opinion, entertained by some writers, 
as to the growth of the spinous processes of the Polycystina 
taking place altogether externally to the soft parts of the 
animal, a tenable one. 
Now, in the concentrically formed subdivision of the Poly- 
cystina (namely, that which, in the classification about to be 
ofiered, I term the Cyclodinal) we find certain plans of growth 
which, at first sight, might be regarded as exceptional. I 
allude to those cases in which the skeleton is not spherical 
^ The Barbadoes earth also contains the " Coccoliths" detected by Huxley 
in the material of the Soundings, and by him regarded as inorganic in tlieir 
nature. They were subsequently found by me to be but portions of other 
structures, namely, of certain spherical cells, to which I gave the name of 
Coccospheres, and which appear to be connected, in some way, with the re- 
production both of the Foraminifera and Polycystine. I have also met 
with them as free-floating organisms in tropical seas. 
