M. J. Frenzel on the Mesozoon Salinella. 83 
the body-wall, new and much smaller cells arise, leaving a 
cavity in the ‘interior which probably subsequently becomes 
the intestinal canal. ‘This process is ushered in by a splitting 
of the nucleus into two, which is indeed to be regarded as an 
indirect division, but which differs materially from initosis, 
Further segments are subsequently produced, which then pass 
to the periphery of the larval cell. What afterwards happens 
I was unfortunately unable to observe. Probably, however, 
as already stated, a cell is formed round each of those nuclet 
—ventral cells on the ciliated ventral side of the larva, dorsal 
cells on the dorsal side, which is beset with sete, and so on. 
Simultaneously an oral opening must be developed at the 
anterior pole and an anal opening posteriorly, and likewise a 
coating of cilia on the inner side of the young cells. With 
this the fully-developed animal would then be constituted, 
and the original digestive cavity of the unicellular larva, 
which, indeed, is filled with endoplasm and is not hollow, 
would have passed into the alimentary canal, which for its 
part is now free from such contents. 
It appears to me that this last circumstance becomes of 
quite critical importance ; for were the intestine also to contain 
a (digestive) plasma, this must be of a cellular nature, and 
must, since particles of food are taken in, digest by the ¢nétra- 
cellular method. But then a multilamellar structure also 
would be already in existence. 
I have already published a preliminary communication 
upon the structure of Salinella*, and have recently sent to 
the press a detailed paper upon the subject. I may be per- 
mitted to refer the reader to the latter for particulars, since 
here it was only intended specially to indicate the closeness 
of the relations between Salinella and the Metazoa from the 
point of view of physiology, and to show that it must not be 
regarded offhand as having arisen from a Protozoon colony, 
although its larval form indeed looks just like a Ciliated 
Infusorian ; for it is precisely the further development of this 
larva, incomplete though my study of ‘it was, which proves 
that it does not develop into the perfect animal by means of 
ordinary division, much as a colony is formed from a single 
Choanoflagellate, but by a far more complicated process, which 
we may most fitly term endogenous cell-formation. 
In conclusion, it is for the present pretty much a matter of 
indifference whether we assign Salinella to the Protozoa or 
to the Metazoa, or introduce it between the two as a Meso- 
zoon, where its position will be quite as disconnected as that 
* Zool. Anzeiger, 1891, no. 367, p. 280 et segg. See p. 109. 
G* 
