Mr. T. H. Huxley on the genus ThalassicoUa. 439 



To come to a similar understanding of the nature of the Tha- 

 lassicoUa nucleata, it is necessary to recur again to certain general 

 characteristics of the reproductive processes in the unicellular 

 animals. 



If we except Tethya, a sponge*, the ordinaiy reproductive 

 elements have as yet been found in no unicellular animal. 



Fission occurs in all except perhaps the Gregarinidse. Gem- 

 mation appears to take place in the Foraminifera and Infusoria. 

 In the Sponges the so-called ova or gemmules seem to be oidy 

 a temporary locomotive condition of the cells, such as occvirs 

 in the Vorticelhs among the Infusoria, and the Protococci among 

 plants. 



But in all {except the Foraminifera) a process of multiphcation 

 by endogenous development occurs, and would seem in some 

 cases to represent sexual pi'opagation. Now the mode of this 

 endogenous multiplication presents remarkable featui-es of simi- 

 larity in the Infusoria, the Gregarinidae, and the Sponges. 



There is a certain period in the existence of Vaginicola cry- 

 stallina, when, gorged with food stored up in the shape of fat gra- 

 nules, &c., within the cavity of its cell-body, it becomes sluggish 

 and eventually still. The body contracts and becomes rounded, 

 and the transparent case closes in and seals up its inhabitant. 

 Eventually long processes are developed from the body, and it 

 takes on the form of the genus Acineta of Ehrenberg. After a 

 while a new life stirs within this chiysahs-like form, and the 

 contained mass gives rise successively (by a sort of fission) to 

 young cihated bodies, which leave the Acineta and become Vagi- 

 nicohs. 



In a similar manner Vorticella microstoma becomes Podophrya 

 fixa ; but sometimes the changed Vorticella has no stalk, and then 

 is the Actinophrys of some authors (not A. Sol). It is not kno^vn 

 in what way the embryos are brought forth here, but it is a very 

 significant fact that both the stalked and unstalked forms have 

 been observed to conjugate. 



Epistylis presents similar phsenomena. 



The Actinophrys Sol, to which more particular reference will 

 be made by and by, has been observed to conjugate, but it is 

 not absolutely known to arise by the metamorphosis of any Vor- 

 ticella, though there is every probability in favour of the suppo- 

 sition that it does. 



The Gregarinidae pass through similar changes. Two forms 



der-like skeleton of certain Foraminifera is extremely like in its appearance 

 to a fragment of the shell of an Echinus, or to the plates contained in the 

 integument of a Holothuria, and we know that these begin with a network 

 of spicules. Consequently there is not by any means so great a distinction 

 between the spicular skeleton of a sponge" and the cuUeuder-like skeleton of 

 an Orbitolina as might at first sight appear." 



* See Annals of Nat. History, S. 2. vol. vii. p. 370. 



