THE INFUSOBIA AND PROTOZOA. 83 



At present the Infusoria are usually regarded as forming 

 part of the same sub-kingdom as the Spongioid, Bhizopoda, and 

 Gregarinida, and as closely allied to them. But, so far as I am 

 aware, no definition can be framed which will yield characters at 

 once common to, and distinctive of, all these four groups ; while 

 recent discoveries tend to widen so greatly the hiatus between 

 the Infusoria and the other three classes, that I greatly doubt if 

 the sub-kingdom Protozoa can be retained in its old sense. 



But if the Infusoria be excluded from it, the remaining 

 groups, notwithstanding the imperfection of our knowledge 

 regarding some of them, exhibit a considerable community of 

 partly negative and partly positive characters. 



The Spongida, Bhizopoda, and Gregarinida^ in fact, are all 

 devoid of any definite oral aperture ; a considerable extent, and 

 sometimes the whole, of the outer surface of the body acting as 

 an ingestive apparatus. Furthermore, the bodies of these ani- 

 mals, or the constituent particles of the compound aggregations, 

 such as the Sponges, exhibit incessant changes of form — the 

 body wall being pushed out at one point and drawn in at another 

 — to such an extent, in some cases, as to give rise to long lobate, 

 or filamentous, processes, which are termed " pseudopodia," 



Finally, all these classes agree in the absence of any well- 

 defined organs of reproduction, innervation, or blood circulation. 



In my first lecture upon Classification, I passed very briefly 

 over the class Bhizopoda, intending to return to the discussion of 

 its limits, and of the value of its subdivisions, when discussing 

 the subdivisions of classes generally. But as time will not 

 permit me to enter at any length upon the greater part of this 

 branch of my subject, I will content myself with briefly stating 

 the conclusions at which I have arrived from a careful study of 

 the extant literature of the subject, combined with some old 

 investigations of my own. 



It appears that three, or perhaps four, types of structure 

 obtain among the Bhizopoda — 



1st. That of the Amoebae— Bhizopods with usually short 

 pseudopodia, a nucleus, and a contractile vesicle. 



2nd. That of the Foraminifera — Bhizopods devoid of nuclei 



G 2 



