On the Structure and Development of Myriotlicla. 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 ch(*mical 

 solution that removed the Foraniiniferal 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, Allmax, P.E.S. 



The encloderm of the body is composed of numerous layers of 

 large spherical cells composed of clear protoplasm, enclosing a 

 nucleus %%-ith some brown granules and refringent corpuscles. 

 Externally it is continued in an altered form into the tentacles, 

 M'hile 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, Ulled with 

 opaque brown granules. Where the eudoderm passes into the 

 tentacles it loses its large clear-celled condition, and consists of 

 small round cells, so loaded \nth opaque granides that the axis of 

 the tentacle appears nearly white under reflected light. 



The free surt'ace 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 podia, which are being constantly projected and withdrawn.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 

 fihrillated layer. It is extremely well developed, and consists of 

 longitudinal muscular fibrillae, closely adherent to the outer sur- 

 face of a structureless hyaline membrane — the " Stiitzlamelle " of 

 Eeichert. 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- 



* It 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." 



