438 Mr. T. H. Huxley on the genus Thalassicolla. 



essentially similar to Amoeba and Arcella, both of which have 

 been shown to be nucleated cells. 



Lastly, we have the Sponges. That the tissue of the Sponges 

 breaks up into masses, each of which is similar to an Amoeba, has 

 been pomted out by Dujardin, and confirmed by Carter and 

 others. Dujardin, however, believing that a peculiar formless 

 substance, " Sarcode," constitutes the tissues of the Sponges (as 

 well as of the Infusoria and many other of the lower animals), 

 fails to point out that they are mere aggregations of true cells. 



This is not the place to discuss the important question, whether 

 the great law developed by Schwann does or does not hold good 

 among the whole of the lower animals. I believe that there is 

 evidence to show that it does ; that everywhere careful analysis 

 will demonstrate the nucleated cell to be the ultimate histolo- 

 gical element of the animal tissues ; and that the " sarcode " of 

 Dujardin, and the " formless contractile substance" of Ecker, are 

 either cells or cell-contents, or the results of the metamorphosis 

 of cells. Be this as it may, however, I can say positively, as the 

 result of recent careful examination, that Spongilla, Halichondria, 

 and Grantia are entirely composed of nucleated cells. 



The Foraminifera and Sponges then, no less than the Infu- 

 soria and Gregarinidse, are " unicellular " animals — animals, that 

 is, which either consist of a single cell, or of definite aggregations 

 of such cells, none of which possesses powers or functions differ- 

 ent from the rest. 



Using the word " unicellular " in this extended sense (as it 

 has been used byNageli and others with regard to the Algse),it 

 may be said that there are four families of unicellular animals ; 

 in two of these, the Infusoria and Gregarinidse, the cells are iso- 

 lated ; in two, the Foraminifera and Sponges, they are aggregated 

 together. 



From these considerations it appears to me that the zoological 

 meaning and importance of the Thalassicolla punctata first be- 

 come obvious. It is the connecting link between the Sponges 

 and the Foraminifera. Allied to the former by its texture and 

 by the peculiar spicula scattered through the substance of some 

 of its varieties, it is equally connected with the latter by the 

 perforated shell of other kinds. If it be supposed that a Thalas- 

 sicolla becomes flattened out, and that a deposit takes place not 

 only round the cells, but between the partitions of the central 

 " vacuolse," it becomes essentially an Orbitoides^. 



* Dr. Carpenter, to whom I communicated these observations, writes to 

 me : " As far as I can understand them, the bodies described (if perfect non- 

 embrj'onic forms) seem to constitute that kind of connecting link between 

 Sponges and Foraminifera, which the relative position I have assigned to 

 them would lead me to expect. It is interesting to remark that the cullen- 



