On the Structure and Development of Myriothela. 297 
to give of the disappearance of the shells from the green sand 
brought up by the ‘Challenger’ in the course of the Agulhas 
Current; but whether it was mechanical abrasion or chemical 
solution that removed the Foraminiferal shells whose internal 
casts formed the Greensand deposit of the Cretaceous epoch, 
must remain for the present an open question*. 
February 11, 1875.—Joseph Dalton Hooker, C.B., President, in 
the Chair. 
* On the Structure and Development of Myriothela.” 
By Prof. Aruman, F.R.S. 
The endoderm of the body is composed of numerous layers of 
large spherical cells composed of clear protoplasm, enclosing a 
nucleus with some brown granules and refringent corpuscles. 
Externally it is continued in an altered form into the tentacles, 
while internally it forms long thick villus-like processes which 
project into the cavity of the body. Towards the free ends of 
these processes there are abundantly developed among the large 
clearer cells, smaller, easily isolated spherical cells, filled with 
opaque brown granules. Where the endoderm passes into the 
tentacles it loses its large clear-celled condition, and consists of 
small round cells, so loaded with opaque granules that the axis of 
the tentacle appears nearly white under reflected light. 
The free surface of the endoderm carries, at intervals, long, very 
slender, sluggishly vibrating cilia, and is overlaid with a thin layer of 
homogeneous protoplasm, which on the villus-like processes becomes 
especially distinct, and which here develops minute mutable pseudo- 
podia, which are being constantly projected and withdrawn. Indeed 
the vibratile cilia appear to be but a modification of these pseudo- 
podial processes of protoplasm. 
Interposed between the endoderm and the ectoderm is the 
fibrillated layer. It is extremely well developed, and consists of 
longitudinal muscular fibrille, closely adherent to the outer sur- 
face of a structureless hyaline membrane—the “ Stiitzlamelle” of 
Reichert. The fibrillated layer, with its supporting membrane, is 
so strong as to remain entire in a section of the animal after the 
tissues on both sides of it have been broken down. 
The ectoderm is composed of two zones, a superficial and a deep. 
The superficial zone consists mainly of two or three layers of 
small round cells containing yellowish granules. Among these 
cells the thread-cells may be seen, lying chiefly near the outer 
surface of the body. Two forms of thread-cells may be here di- 
* Tt is due to Prof. W. C. Williamson to point out that, in the Memoir 
already referred to, he indicated the probability “that many of our European 
Greensands, and other siliceous strata, however barren of such structures they 
appear, may have once contained multitudes of calcareous microscopic organisms, 
some of which have been removed after the consolidation of the strata, either 
_leaving hollow casts, or having had the cavities subsequently filled with silica.” 
