642 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Porifera. 
Origin of Triradiate Spicules of Leucosolenia.* — Mr. E. A. Mincliin 
found that in Leucosolenia coriacea the youngest spicules are surrounded 
by six cells which are similar, in all their characters, to the cells of the 
external flat epithelium of the sponge. Three cells of the external 
epithelium wander inwards, and give rise by division to six cells which 
are so arranged that three are placed more internally and three more 
externally. The spicule is formed by the three inner cells, a ray being 
formed by each. The three outer cells soon lose their rounded form, 
and by throwing out processes assume an amoeboid appearance. The 
three inner cells, however, alone secrete the rays, and continue to do so 
until the spicule is full grown. The rays soon appear to project beyond 
their formative cells, but they are in reality covered by a thin layer of 
protoplasm. At the same time, the spicule sheath makes its appearance 
as a denser layer of substance between the protoplasm of the formative 
cell and the calcareous spicule, and it is by continued calcification of 
the sheath that the spicule grows. The spicule rays attain their full 
thickness at their bases before they reach their full length. The forma- 
tive cells remain at the base until it is built up to its full thickness. 
Each cell then migrates along its ray towards the tip, building up the 
ray to its full thickness as it goes. In the fully formed ray the forma- 
tive cell is found adherent to the extreme tip. Mr. Minchin points out 
that the origin of the spicule-forming cells from the external flat 
epithelium is another nail in the coffin of the so-called mesoderm. In 
these forms sponges are to be regarded as two-layered animals, composed 
of a dermal and a gastral layer. The former is differentiated into (1) an 
external flat, contractile epithelium, the neuro-muscular system, and 
(2) an internal connective- tissue layer. The gastral layer consists of 
the collar cells, and perhaps also of the amoeboid wandering cells. The 
history of the triradiate spicule shows that it must be regarded as having 
been derived from the fusion of three originally separate monactinal 
spicules. This supports the theory of Schulze that the triradiates of 
the more primitive Ascons arose as an adaptation to the structure of the 
sponge, and goes against Dreyer’s theory that the primitive spicule of 
all sponges is a tetraxon, a form which he says is the direct mechanical 
outcome of the vesicular structure of living bodies. 
Collar Cells of Heteroccela.j — Mr. Gr. Bidder, wffio prefaces his paper 
with a useful summary of its contents, commenced his investigations at 
Naples on Leucandra aspera and Sycon rapJianus, on which he made 
feeding experiments. Observations on living cells were made chiefly on 
Sycon compressum at Plymouth. The last named species is the best 
suited of all known to the author for examination under high powers 
during life, for its collar cells are among the largest, if not as large as 
any known. As it appears to Mr. Bidder to be inconvenient to use 
names for the tissues of sponges which connote comparison with other 
groups of multicellular animals, he employs the following terms. An 
“ ectocyte ” is any cell forming part of the external surface of a sponge, 
including the afferent system of canals. A “ mesocyte ” is a parenchym 
* Proc. Eoy. Soc., lviii. (1895) pp. 204 and 5. 
f Quart. Journ. Micr. Sci., xxxviii. (1895) pp. 9-43 (1 pi.). 
