Mr. T. H. Huxley on the genus TlialassicoUa. 437 



with the Diatomaceae and Desmidiae, the Volvocina, the Mona- 

 dina, the Vibriones, and to these we must very probably add the 

 Astasisea. 



So utter has been the want of critical discrimination in the 

 construction of genera and species, that Cohn, in his admirable 

 memoir upon Protococcus pluvialis, enumerates among the twenty- 

 one forms (to which distinct names have been given by authors) 

 assumed by the Protococcus, no less than eight of Prof. Ehren- 

 berg's genera. The family " Polygastria," thus cut down to less 

 than one-half its original dimensions, contains none but animals 

 which are either simple nucleated cells, or such cells as have 

 undergone a certain amount of change, not sufficient however 

 to destroy their real homology with nucleated cells. 



A nucleus has been found in Euglena, Arcella, Amceba, Am- 

 phileptus, TracheUus, Bursaria, Pai'amacium, Nassula, Chilodon, 

 Oxytricha, Stylonichia, Stentor, Vorticella, Euplotes, Trichodina, 

 Loxodes, and other genera. It may be brought out by acetic 

 acid just like any other nucleus in Vorticella and Euglena. 



The animal is an unchanged cell in Euglena, in Amceba and in 

 Opalina. In others, as the Vorticella, there is a more or less di- 

 stinct permanent cavity in the interior of the cell which opens 

 externally, an occurrence not without parallel among the secre- 

 ting cells of insects. Certain genera, such as Nassula, have an 

 armature of spines, but so have some of the Gregarinidse which 

 are unquestionably simple cells. 



Contractile spaces, — cavities which appear and disappear in dif- 

 ferent parts of the Infusoria, and sometimes become filled with 

 the ingesta, — are found no less commonly in the component cells 

 of the tissues of many of the lower animals, and according to 

 Cohn in the primordial cells of plants also. 



The " Polygastria,^' then, may be justly considered to be sim- 

 ple cells, and to form a type perfectly comparable with Thalas- 

 sicolla. 



The researches of Henle, Stein, and Kolliker have made us 

 acquainted with another form of cellular animals — the Gregari- 

 nidfe. 



These are nucleated cells, without cilia, but with contractile 

 walls, which lead an independent parasitic life in the intestines 

 of many of the Invertebrata, principally insects. 



The Gregarinidse, like the Infusoria, are generally, if not in- 

 variably, single, solitary cells. 



A third type is formed by the Foraminifera. The fate of these 

 animals is somewhat singular. Considered to be Cephalopoda 

 by D'Orbigny; Brj'ozoa by Ehrenberg; rudimentary Gasteropoda 

 by Agassiz ; all careful observation tends to confirm the opinion 

 of Dujardin, that the fabrication of their remarkable shells is 



