clxvi Introduction. 



Radiolaria. In all Rhizopoda, except the Amoehina, the pseudo- 

 podia are very numerous, ramify minutely, inosculate by their 

 ramifications, and show in their interior a cyclosis or circulation 

 of granular sarc^ode ; and it is by their suctional agency that nutri- 

 tive matters are absorbed. In the Amoebina, which on this ac- 

 count have been sometimes, as by Kolliker and V. Carus, separated 

 from the Rhizopoda and classed with the Infusoria, the pseudo- 

 podia are few and large, and neither anastomose nor show any 

 cyclosis of granular sarcode in their interior. The exterior layer of 

 the bodies is better defined than that of other Rhizopoda, and they 

 extract nutriment from their food by enveloping it, as indeed the 

 Actinophri/na do also, in the substance of their parenchyma. 



The differentiation of the outer layers or ' ectosarc^ of the Amoe- 

 hina, from the inner or ' endosarc,^ is carried so far in the direction 

 of increase of tenacity and consistence as to render it probable that 

 a definite spot must exist in their external periphery, which acts as 

 an oral inlet. No other Rhizopoda possess either a mouth or anus. 



Reproduction appears to be effected in the simpler Rhizopoda by 

 fission of the protoplasmic mass, of which their bodies are exclusively 

 made up; but in the higher forms of the class, the Amoebina and 

 Actinophri/na, it would appear that true sexual products may be 

 formed. 



The Rhizopoda may be either solitary or social. They are mostly 

 marine, but some Amoebina, ActinopJiryna, and Gromida are found 

 in fresh water. 



Class, Spongiadae. 



Social Protozoa, which form one or many aggregated colonies, 

 each one of which possesses on its exterior a single exhalant ' os- 

 culum,' and a great number of smaller inhalant ' pores -' whence 

 the Class has taken its name of ' Porifera.' The cells which make 

 up each colony, are supported upon an internal skeleton, which 

 may be siliceous or calcareous, horny or leathery in consistence ; 

 Halisnrcina, which has been described as devoid of any skeleton, 

 and as being merely a colony of naked amoebiform bodies, having 

 been shown by Dr. Bowerbank to possess more or less of a siliceous 

 skeleton. The cells are of very various forms in each colony ; some 

 being ciliated, some amoebiform, whilst others may have their sar- 

 codic substance so fused together as to form continuous masses. 



