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



of these creatures are known; the one consisting of protean 

 nucleated cells, the other of motionless spherical sacs, containing 

 a vast number of minute bodies resembling Naviculce in shape, 

 and thence called " Navicella-sacs." Now, according to Stein, 

 although the fact has been doubted by others, the " Navicella- 

 sacs " result from the conjugation of two Gregarince, which have 

 become motionless and filled with an accumulation of granules. 

 ' Certain it is that the Navicellse are developed within the gra- 

 nular mass like embryo-cells within the yelk, and that when freed 

 by the bursting of the Navicella-sac they become Gregarince. 



Lastly, in the freshwater sponge [Spongilla), which consists of 

 an aggregation of nucleated protean cells like a mass of Grega- 

 rince, a certain number of the cells at various points scattered 

 through the substance of the Spongilla become motionless and 

 distended with granules, and receiving first a membranous and 

 then a siliceous investment, constitute the " seed-like bodies." 



From Mr. Carter's account it would appear that when the 

 " seed-like body " germinates its cells burst, and their granular 

 contents become mixed. Subsequently protean cells, like the 

 ordinary sponge-cells, make their appearance pari passu with the 

 disappearance of the granules. 



Supposing this account to be correct, the conjugation in Spon- 

 gilla would be perfectly analogous to that of the Desmidiae and 

 Diatomacese, while in the Infusoria and Gregarinidse it would 

 resemble that of Zygnema. 



Generalizing the above details (full authority for which may 

 be found in the appended list of works), we may say that with 

 the exception of the Foraminifera, about whose reproductive pro- 

 cesses nothing is as yet known, the Protozoa all reproduce their 

 kind by a process of endogenous development which is accom- 

 panied by greater or less changes in the structure and powers 

 of the reproducing cell. We may add that in many cases these 

 changed cells have been observed to conjugate, previous to the 

 occurrence of the endogenous development. 



Bearing all these facts in mind, let us return to Thalassicolla 

 nucleata. If the Th. punctata answer to a mass of sponge-cells or 

 an aggregation of Gregarince, is it not possible that the Th. nu- 

 cleata may answer to the altered reproductive cell ? I have shown 

 that the Th. nucleata may very possibly be nothing more than a 

 separated and enlarged cell of Th. punctata, and this possibility 

 upon structural grounds becomes, I think, converted into proba- 

 bility, if Th. nucleata be compared with Actinophrys Sol, which 

 there is every reason to believe is the reproductive stage of one 

 of the Vorticellinse. 



Actinophrys Sol is a spherical gelatinous mass consisting of an 

 internal dark granular portion and a clearer external zone from 

 which many radiating threads are given off. Vacuolse are scat- 



