M. J. Frenzel on the Mesozoon Salinella. 83 



tlie body-wall, new niid much smaller cells arise, leaving a 

 cavity in the interior wliicli 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 mitosis. 

 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 nuclei 

 — ventral cells on the ciliated ventral side of the larva, dorsal 

 cells on the dorsal side, which is beset with setae, 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 endoplasra 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 intra- 

 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 ])ress a detailed paper upon the subject. I may be per- 

 mitted to refer tlie 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. 230 et seqq. See p. 109. 



6* 



